Ravi can repair a road stretch in 20 days and Sunita in 30 days. They work together for 6 days, after which Ravi is moved to another site and Sunita finishes the rest alone. How many days does the whole repair take?

Explanation

1. 01

   Together they do $\frac{1}{20} + \frac{1}{30} = \frac{1}{12}$ per day.

2. 02

   In 6 days: $6 \times \frac{1}{12} = \frac{1}{2}$ of the road.

3. 03

   Sunita does the other $\frac{1}{2}$ at $\frac{1}{30}$ per day: $\frac{1}{2} \times 30 = 15$ days.

4. 04

   Total = $6 + 15 = 21$ days.