Three workers, Salim, Tanya and Uday, can build a shed alone in 30, 40 and 60 days. All three begin together, but after 4 days Uday leaves. How many more days do Salim and Tanya need to finish the shed?

Explanation

1. 01

   All three: $\frac{4 + 3 + 2}{120} = \frac{9}{120} = \frac{3}{40}$ per day.

2. 02

   In 4 days: $\frac{12}{40} = \frac{3}{10}$ done, so $\frac{7}{10}$ remains.

3. 03

   Salim + Tanya: $\frac{4+3}{120} = \frac{7}{120}$ per day.

4. 04

   Time: $\frac{7}{10} \div \frac{7}{120} = 12$ days.