Here, (25 men + 10 boys) × 6 = (21 men + 30 boys) × 5
⇒ 150 men + 60 boys = 105 men + 150 boys
⇒ 45 men = 90 boys Hence, 1 man = 2 boys
25 men + 10 boys = 50 boys + 10 boys = 60 boys
40 men = 80 boys
In 6 days, the whole work has been done by 60 boys.
Hence, in 4 days, the whole work will be done by
= 90 boys.
Therefore, required number of boys = 90 – 80 = 10 boys.