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Re: The sum of three integers A, B and C is 120. A is one third of the sum [#permalink]
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Carcass wrote:
The sum of three integers A, B and C is 120. A is one third of the sum of B and C and B is one fifth of the sum of A and C. What is C?

A. 20
B. 30
C. 45
D. 50
E. 70


The sum of three integers A, B and C is 120
We can write: A + B + C = 120

A is one third of the sum of B and C
We can write: A = (1/3)(B + C)
Multiply both sides by 3 to get: 3A = B + C

B is one fifth of the sum of A and C.
We can write: B = (1/5)(A + C)
Multiply both sides by 5 to get: 5B = A + C

There are MANY different ways to solve this system of 3 equations.
Here's ONE solution.

Take A + B + C = 120 and replace B + C with 3A to get: A + 3A = 120
Solve to get A = 30

Take A + B + C = 120 and replace A + C with 5B to get: B + 5B = 120
Solve to get B = 20

Now that we now A = 30, and B = 20, AND we know that A + B + C = 120, we can conclude that C = 70

Answer: E

Cheers,
Brent
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Re: The sum of three integers A, B and C is 120. A is one third of the sum [#permalink]
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Re: The sum of three integers A, B and C is 120. A is one third of the sum [#permalink]
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