3 men can do a piece of work in 18 days. 6 children can also do that work in 18 days. 4 men and 4 children together will finish the work in how many days?

3 men = 6 children
⇒ 1 man = 2 children
∴ 4 men + 4 children = 4 men + (4/2) men = 6 men
Given, M₁ = 3, M₂ = 6, D₁ = 18, W₁ = W₂ = 1 and D₂= ?
According to the formula,
M₁D₁W₂ = M₂D₂W₁
⇒ 3 x 18 x 1 = 6 x D₂ x 1
∴ D₂ = 3 x 18/6 = 9 days