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 78kg instead of 87 kg. The correct average weight is
Total weight with misreading = $$20 \times 89.4 = 1788$$
(87-78=9) should be added to correct the total = 1788+9 = 1797
Hence correct average will be $$\frac{1797}{20} = 89.85$$
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