PSAT Math Multiple-Choice Question 345: Answer and Explanation

Question: 345

What is the value of x if 25x - 7y = 28 and 10x + 7y + 18 = 60?

• A.
• B.
• C. 2
• D.

Correct Answer: C

Explanation:

C

Difficulty: Medium

Category: Combination

Strategic Advice: The numbers here are fairly large, so substitution is not likely to be convenient. Moreover, the y-coefficients have the same absolute value, so combination will likely be the faster way to solve.

Getting to the Answer: Start by writing the second equation in the same form as the first, then use combination to solve for x:

Thus, (C) is correct.

If you feel more comfortable using substitution, you can maximize efficiency by solving one equation for 7y and substituting that value into the other equation:

Note that the arithmetic is fundamentally the same, but the setup using combination is quicker and visually easier to follow.