Tamara J. answered • 11/15/12

Math Tutoring - Algebra and Calculus (all levels)

(1.) In writing a system of equations, first note what you are looking to solve for. In this problem, you are asked to find how many dimes and how many quarters are needed to make up a total amount of $6.35 given that you have a total of 35 coins.

Let D represent the number of dimes and Q represent the number of quarters. From this, you can generate the following equation:

D + Q = 35

Since the value of a dime is $0.10 and that of a quarter is $0.25, then the sum of the # of dimes times the value of a dime (0.10) and the # of quarters times the value of a quarter (0.25) equals the total amount of $6.35. That is,

(0.10)D + (0.25)Q = 6.35

Thus, the system of equations that will solve for the problem contains the following equations:

D + Q = 35

(0.10)D + (0.25)Q = 6.35

(2.) When solving a system of equations with 2 variables (i.e., D and Q), your goal is to eliminate one of the variables in order to solve for the other variable. Once you've solved for one of these variables, you can use it to solve for the other variable by plugging it into one of the original equations.

First, pick a variable to eliminate. I will choose to eliminate Q to demonstrate. If I multiply the second equation by -4 and add the 2 equations together, I will have eliminated Q and, therefore, be able to solve for D:

-4*[(0.10)D + (0.25)Q = 6.35] ==> (-0.40)D + (-1) Q = -25.4

==> (-0.40)D - Q = -25.4

Add the new equation to the first equation:

D + Q = 35

+ (-0.40)D - Q = -25.4

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D - (0.40)D + Q - Q = 9.60 ==> (0.60)D = 9.60

Divide both sides by 0.60 to solve for D:

(0.60)D / (0.60) = 9.60 / 0.60 ==> D = 16

Plug 16 in for D in the first equation to solve for Q:

D + Q = 35 ==> 16 + Q = 35

Subtract 16 from both sides of the equation:

16 + Q = 35

-16 -16

_________________

Q = 19

You can check your work by plugging in these values for D and Q into the second equation:

(0.10)D + (0.25)Q = (0.10)16 + (0.25)19 = 1.60 + 4.75 = 6.35

Thus, Pam has 16 dimes and 19 quarters.