The average weight of a group of 20 boys was calculated to be 89.4 kg and it was later discovered that one weight was misread as 78 kg instead of 87 kg. The correct average weight is -

According to the question,
Average weight of a 20 boys = 89.4 kg
Sum of a weight of 20 boys = 89.4 × 20 = 1788 kg
It was later discovered that one weight was misread as 78 kg instead of 87 kg.
∴ Difference = 87 - 78 = 9 kg
∴ Actual sum of a weight of 20 boys
= 1788 + 9
= 1797 kg.
Actual average = $$\frac{1797}{20}$$   = 89.85 kg