Let initially x men are there to complete work in 2y days. Now after half of work completed, i.e. after y days, double men as before joined the group. This means now there are 3x men working. So after y days Ml x D1 =M2 x D2 x × y=3x × (y - 6) ⇒ y=9 So total days x men would have taken = 2y = 18 days