Pipe A can fill a cistern in 8 hours and pipe B in 12 hours. Both are opened, and after 2 hours pipe A is shut. How many more hours does pipe B take to fill the cistern?

Explanation

1. 01

   Both together: $\frac{1}{8} + \frac{1}{12} = \frac{5}{24}$ per hour.

2. 02

   In 2 hours: $\frac{10}{24} = \frac{5}{12}$ filled, leaving $\frac{7}{12}$.

3. 03

   B alone: $\frac{7}{12} \times 12 = 7$ hours.