Suppose you took eight math tests this semester. If your average score on your first six
tests was 84 and your average score on all eight tests was 86, then what was the average
of your last two test scores?
If your anverage score on your first six test was 84, then you had 6 x 84 = 504 points.
If you average score on all eight tests was 86, then you had 8 x 86 = 688 points.
So, you must have gotten 688 - 504 = 184 points on your last two tests.
184 points on the last two tests gave you an average of 184 / 2 = 92 on the last two tests.