1985 AJHSME Problem 17

Problem

If your average score on your first six mathematics tests was $84$ and your average score on your first seven mathematics tests was $85$, then your score on the seventh test was

$\text{(A)}\ 86 \qquad \text{(B)}\ 88 \qquad \text{(C)}\ 90 \qquad \text{(D)}\ 91 \qquad \text{(E)}\ 92$

In-depth Solution by BoundlessBrain!

https://youtu.be/EIoCLu2wR88

Solution

The total score of your first 6 tests was $84 \cdot 6 = 504$ and the total score of your first 7 tests was $85 \cdot 7 = 595.$ So, the missing score is $595-504 = \boxed{\text{(D)} \quad 91}.$