Pipes A and B fill a tank in 12 and 18 minutes. Both are opened together, and A is closed after 4 minutes. How much longer does B take to fill the tank?

Explanation

1. 01

   In 4 minutes both fill $4\left(\frac{1}{12} + \frac{1}{18}\right) = \frac{5}{9}$.

2. 02

   B fills the remaining $\frac{4}{9}$ in $\frac{4}{9} \times 18 = 8$ minutes.