Math Challenge: what is the other number in terms of x?

GRE Math Challenge Question

If the average (arithmetic mean) of two numbers is 30 and one of the numbers is x, what is the other number in terms of x?

A) 60 - x
B) 60 – 2x
C) 30 - x
D) (30 – x) / 2
E) X - 60

Solution:

Correct Choice : A

We know that sum of terms/number of terms = average, so set up the equation as given and solve.
Let's call the number we want y:

(y + x)/2 = 30 => multiply both sides by 2
(y + x) = 60 => subtract an x from both sides
y = 60 - x.

The answer is A.

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