Explanation:
(seven numbers total) / 7 = 81
seven numbers total = 7 * 81 = 567
NOW REMOVE ONE OF THE NUMBERS
( Sum of six numbers) / 6 = 78
sum of six numbers = 6 * 78 = 468
sum of seven numbers - sum of six numbers = the number removed
567 - 468 = 99 <===number removed