Average weight of a group of 8 boys is 89 Kg. Later it was found that the weighing machine was showing 4 Kg more than the actual weight. What is the actual average weight?
Let the total weight of the group as measured with the faulty machine be x. Then, by average formula $89 = x/8$, which gives x = $89 × 8 = 712$. When weight of each of the 8 boys is reduced by 4 Kg, the new total becomes $712 - 8 × 4 = 680$, the new average becomes $680/8 = 85$. TIP: As a shortcut, the new average = old average - error in weighing machine.