GMAT Quantitative Reasoning Practice Test 1

This is an average problem, so use the average formula. If the average of 7 numbers is $12.2$ , we can solve for their sum: $7\times 12.2=85.4$ . If four of these numbers total 42.8, then by subtracting 42.8 from 85.4, we get the sum of the other three numbers, 42.6. To find the average of these three numbers, we divide their sum by their number: $\frac{42.6}{3}=14.2$.