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Systems of Linear Equations and Problem Solving
Read and reread the problem.  Suppose that the second number is 5.  Then the first number, which is 4 more than twice the second number, would have to be 14 (4 + 25).  Is their total 25?  No:  14 + 5 = 19.  Our proposed solution is incorrect, but we now have a better understanding of the problem.Since we are looking for two numbers, we letx = first numbery = second numberFinding an Unknown NumberExample:One number is 4 more than twice the second number.  Their total is 25.  Find the numbers.1.)  UnderstandContinued
One number is 4 more than twice the second number.Their total is 25.Finding an Unknown NumberExample continued:2.)  Translatex = 4 + 2yx + y = 25Continued
Finding an Unknown NumberExample continued:3.)  SolveWe are solving the system        x = 4 + 2y    and x + y = 25Using the substitution method, we substitute the solution for x from the first equation into the second equation.x + y = 25 (4 + 2y) + y = 25Replace x with result from first equation.4 + 3y = 25Simplify left side.3y = 21Subtract 4 from both sides and simplify.y = 7Divide both sides by 3.Now we substitute the value for y into the first equation.Continuedx = 4 + 2y = 4 + 2(7) = 4 + 14 = 18
Finding an Unknown NumberExample continued:4.)  InterpretCheck:  Substitute x = 18 and y = 7 into both of the equations.	First equation,x = 4 + 2y 18 = 4 + 2(7)      true	Second equation,x + y = 2518 + 7 = 25        trueState:   The two numbers are 18 and 7.
Solving a Problem about PricesExample:Hilton University Drama club sold 311 tickets for a play.  Student tickets cost 50 cents each; non-student tickets cost $1.50.  If total receipts were $385.50, find how many tickets of each type were sold.1.)  UnderstandRead and reread the problem.  Suppose the number of students tickets was 200.  Since the total number of tickets sold was 311, the number of non-student tickets would have to be 111 (311  200).Continued
Solving a Problem about PricesExample continued:1.)  Understand (continued)Are the total receipts $385.50?  Admission for the 200 students will be 200($0.50), or $100.  Admission for the 111 non-students will be 111($1.50) = $166.50.  This gives total receipts of $100 + $166.50 = $266.50.  Our proposed solution is incorrect, but we now have a better understanding of the problem.Since we are looking for two numbers, we lets = the number of student ticketsn = the number of non-student ticketsContinued
Hilton University Drama club sold 311 tickets for a play.total receipts were $385.50Admission for studentsTotal receiptsAdmission for non students+=385.501.50nSolving a Problem about PricesExample continued:2.)  Translates + n = 3110.50sContinued
s + n = 311s + n = 311s  3n = 7712(0.50s + 1.50n) = 2(385.50)Solving a Problem about PricesExample continued:3.)  SolveWe are solving the system   s + n = 311  and   0.50s + 1.50n = 385.50Since the equations are written in standard form (and we might like to get rid of the decimals anyway), well solve by the addition method.  Multiply the second equation by 2.simplifies to2n = 460n = 230Now we substitute the value for n into the first equation.s + n = 311s + 230 = 311s = 81Continued
Solving a Problem about PricesExample continued:4.)  InterpretCheck:  Substitute s = 81 and n = 230 into both of the equations.	First equation,s + n = 31181 + 230 = 311      true	Second equation,	  0.50s + 1.50n = 385.50 0.50(81) + 1.50(230) = 385.50	     40.50 + 345 = 385.50      trueState:   There were 81 student tickets and 230 non-student tickets sold.
price per unitnumber of units=price of all unitsSolving a Mixture ProblemExample:A Candy Barrel shop manager mixes M&Ms worth $2.00 per pound with trail mix worth $1.50 per pound.  Find how many pounds of each she should use to get 50 pounds of a party mix worth $1.80 per pound.1.)  UnderstandRead and reread the problem.  We are going to propose a solution, but first we need to understand the formulas we will be using.  To find out the cost of any quantity of items we use the formulaContinued
Solving a Mixture ProblemExample continued:1.)  Understand (continued)Suppose the manage decides to mix 20 pounds of M&Ms.  Since the total mixture will be 50 pounds, we need 50  20 = 30 pounds of the trail mix.  Substituting each portion of the mix into the formula,M&Ms         $2.00 per lb    20 lbs   =   $40.00trail mix        $1.50 per lb    30 lbs   =   $45.00Mixture         $1.80 per lb    50 lbs   =   $90.00Continued
Solving a Mixture ProblemExample continued:1.)  Understand (continued)Since $40.00 + $45.00  $90.00, our proposed solution is incorrect (hey, we were pretty close again), but we now have a better understanding of the problem. Since we are looking for two quantities, we letx = the amount of M&Msy = the amount of trail mixContinued
Fifty pounds of party mixprice per unitprice of all unitsUsingnumber of units=Price of M&MsPrice of trail mixPrice of mixture+=2xSolving a Mixture ProblemExamplecontinued:2.)  Translatex + y = 501.5y1.8(50) = 90Continued
3(x + y) = 3(50)  3x + 3y = 150 4x  3y =  1802(2x + 1.50y) = 2(90)Solving a Mixture ProblemExample continued:3.)  SolveWe are solving the system   x + y = 50   and   2x + 1.50y = 90Since the equations are written in standard form (and we might like to get rid of the decimals anyway), well solve by the addition method.  Multiply the first equation by 3 and the second equation by 2.simplifies to x     =  30x = 30Now we substitute the value for x into the first equation.Continuedx + y = 5030 + y = 50y = 20
Solving a Mixture ProblemExample continued:4.)  InterpretCheck:  Substitute x = 30 and y = 20 into both of the equations.	First equation,x + y = 5030 + 20 = 50      true	Second equation,	      2x + 1.50y = 90       2(30) + 1.50(20) = 90	          60 + 30 = 90      trueState:   The store manager needs to mix 30 pounds of M&Ms and 20 pounds of trail mix to get the mixture at $1.80 a pound.
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Applications of Systems

  • 1. Systems of Linear Equations and Problem Solving
  • 2. Read and reread the problem. Suppose that the second number is 5. Then the first number, which is 4 more than twice the second number, would have to be 14 (4 + 25). Is their total 25? No: 14 + 5 = 19. Our proposed solution is incorrect, but we now have a better understanding of the problem.Since we are looking for two numbers, we letx = first numbery = second numberFinding an Unknown NumberExample:One number is 4 more than twice the second number. Their total is 25. Find the numbers.1.) UnderstandContinued
  • 3. One number is 4 more than twice the second number.Their total is 25.Finding an Unknown NumberExample continued:2.) Translatex = 4 + 2yx + y = 25Continued
  • 4. Finding an Unknown NumberExample continued:3.) SolveWe are solving the system x = 4 + 2y and x + y = 25Using the substitution method, we substitute the solution for x from the first equation into the second equation.x + y = 25 (4 + 2y) + y = 25Replace x with result from first equation.4 + 3y = 25Simplify left side.3y = 21Subtract 4 from both sides and simplify.y = 7Divide both sides by 3.Now we substitute the value for y into the first equation.Continuedx = 4 + 2y = 4 + 2(7) = 4 + 14 = 18
  • 5. Finding an Unknown NumberExample continued:4.) InterpretCheck: Substitute x = 18 and y = 7 into both of the equations. First equation,x = 4 + 2y 18 = 4 + 2(7) true Second equation,x + y = 2518 + 7 = 25 trueState: The two numbers are 18 and 7.
  • 6. Solving a Problem about PricesExample:Hilton University Drama club sold 311 tickets for a play. Student tickets cost 50 cents each; non-student tickets cost $1.50. If total receipts were $385.50, find how many tickets of each type were sold.1.) UnderstandRead and reread the problem. Suppose the number of students tickets was 200. Since the total number of tickets sold was 311, the number of non-student tickets would have to be 111 (311 200).Continued
  • 7. Solving a Problem about PricesExample continued:1.) Understand (continued)Are the total receipts $385.50? Admission for the 200 students will be 200($0.50), or $100. Admission for the 111 non-students will be 111($1.50) = $166.50. This gives total receipts of $100 + $166.50 = $266.50. Our proposed solution is incorrect, but we now have a better understanding of the problem.Since we are looking for two numbers, we lets = the number of student ticketsn = the number of non-student ticketsContinued
  • 8. Hilton University Drama club sold 311 tickets for a play.total receipts were $385.50Admission for studentsTotal receiptsAdmission for non students+=385.501.50nSolving a Problem about PricesExample continued:2.) Translates + n = 3110.50sContinued
  • 9. s + n = 311s + n = 311s 3n = 7712(0.50s + 1.50n) = 2(385.50)Solving a Problem about PricesExample continued:3.) SolveWe are solving the system s + n = 311 and 0.50s + 1.50n = 385.50Since the equations are written in standard form (and we might like to get rid of the decimals anyway), well solve by the addition method. Multiply the second equation by 2.simplifies to2n = 460n = 230Now we substitute the value for n into the first equation.s + n = 311s + 230 = 311s = 81Continued
  • 10. Solving a Problem about PricesExample continued:4.) InterpretCheck: Substitute s = 81 and n = 230 into both of the equations. First equation,s + n = 31181 + 230 = 311 true Second equation, 0.50s + 1.50n = 385.50 0.50(81) + 1.50(230) = 385.50 40.50 + 345 = 385.50 trueState: There were 81 student tickets and 230 non-student tickets sold.
  • 11. price per unitnumber of units=price of all unitsSolving a Mixture ProblemExample:A Candy Barrel shop manager mixes M&Ms worth $2.00 per pound with trail mix worth $1.50 per pound. Find how many pounds of each she should use to get 50 pounds of a party mix worth $1.80 per pound.1.) UnderstandRead and reread the problem. We are going to propose a solution, but first we need to understand the formulas we will be using. To find out the cost of any quantity of items we use the formulaContinued
  • 12. Solving a Mixture ProblemExample continued:1.) Understand (continued)Suppose the manage decides to mix 20 pounds of M&Ms. Since the total mixture will be 50 pounds, we need 50 20 = 30 pounds of the trail mix. Substituting each portion of the mix into the formula,M&Ms $2.00 per lb 20 lbs = $40.00trail mix $1.50 per lb 30 lbs = $45.00Mixture $1.80 per lb 50 lbs = $90.00Continued
  • 13. Solving a Mixture ProblemExample continued:1.) Understand (continued)Since $40.00 + $45.00 $90.00, our proposed solution is incorrect (hey, we were pretty close again), but we now have a better understanding of the problem. Since we are looking for two quantities, we letx = the amount of M&Msy = the amount of trail mixContinued
  • 14. Fifty pounds of party mixprice per unitprice of all unitsUsingnumber of units=Price of M&MsPrice of trail mixPrice of mixture+=2xSolving a Mixture ProblemExamplecontinued:2.) Translatex + y = 501.5y1.8(50) = 90Continued
  • 15. 3(x + y) = 3(50) 3x + 3y = 150 4x 3y = 1802(2x + 1.50y) = 2(90)Solving a Mixture ProblemExample continued:3.) SolveWe are solving the system x + y = 50 and 2x + 1.50y = 90Since the equations are written in standard form (and we might like to get rid of the decimals anyway), well solve by the addition method. Multiply the first equation by 3 and the second equation by 2.simplifies to x = 30x = 30Now we substitute the value for x into the first equation.Continuedx + y = 5030 + y = 50y = 20
  • 16. Solving a Mixture ProblemExample continued:4.) InterpretCheck: Substitute x = 30 and y = 20 into both of the equations. First equation,x + y = 5030 + 20 = 50 true Second equation, 2x + 1.50y = 90 2(30) + 1.50(20) = 90 60 + 30 = 90 trueState: The store manager needs to mix 30 pounds of M&Ms and 20 pounds of trail mix to get the mixture at $1.80 a pound.