12 men and 18 boys, working 15/2 hours a day, can do a piece of work in 60 days. If a man works equal to 2 boys, then how many boys will be required to help 21 men to do twice the work in 50 days, working 9 hours a day ?

1 man ≡ 2 boys ⇔ (12 men + 18 boys)
≡ (12 × 2 ×18) boys = 42 boys
Let required number of boys = x
⇒ (21 men + x boys) ≡ (21 × 2 × x) boys = (42 + x) boys
Less days, More boys (Indirect proportion)
More hours per day, Less boys (Indirect proportion)
More work, More boys (Direct proportion)
Days ⇒ 50 : 60
Hours per day ⇒ 9 : 15/2 ⟩⟩⟩ :: 42 : (42 + x)
Work ⇒ 1 : 2
50 × 9 × 1 × (42 + x) = 60 × 15/2 × 2 × 42
⇒ x = 42