GIVEN:
(28 boys + 7 men)'s time = 4 days
(21 boys + 3 men)'s time = 7 days
FORMULA USED:
Work = efficiency × time
CALCULATION:
let boy's efficiency be b and man's efficiency be m.
Then, work = (28b + 7m) × 4
Also, work = (21b + 3m) × 7
Equating both the equations,
(28b + 7m) × 4 = (21b + 3m) × 7
⇒ 112b + 28m = 147b + 21m
⇒ 7m = 35b
⇒
=
∴ Work = (28 × 1 + 7 × 5) × 4 = 252
12 boys' efficiency = 12
12 boys' time =
= 21 days