PSAT Math Grid-Ins Question 113: Answer and Explanation

Question: 113

–3a + 6b = 1.8

4a – 3b = 1.1

The ordered pair (a, b) is a solution to the system of equations above. What is the value of b?

Correct Answer: or 0.7

Explanation:

or 0.7

The question asks for the value of a variable in a system of equations. Solve the system of equations using stacking. When stacking the system of equations, multiply the second equation by 2 and add it to the first equation to eliminate the b variable.

The new equation becomes 5a = 4. Divide both sides by 5 to get a = 0.8. Plug a back into either equation to solve for b. The first equation becomes –3(0.8) + 6b = 1.8, or –2.4 + 6b = 1.8. Add 2.4 to both sides to get 6b = 4.2. Divide both sides by 6 to get b = 0.7. The correct answer is 0.7 or .