Description for Correct answer:
Time taken by A to fill the tank, m = 15 min

Therefore, Time taken by B to fill the tank, n = \( \Large 15 \times 4 \) = 60 min

Therefore, Required time taken \( \Large \frac{m \times n}{m + n} \)

= \( \Large \frac{15 \times 60}{15 + 60}=\frac{15 \times 6}{75} \) = 12 min.