Two taps P and Q can fill an empty tank in 15 hours and 30 hours respectively. Both taps were opened at 4 a.m. and after some time, tap Q was closed. It was found that the tank was full at 4 p.m. At what time was the tap Q shut ?

Explanation

To solve the problem step by step, we will calculate the work done by taps P and Q and determine when tap Q was closed.

### Step-by-Step Solution:

1. **Determine the filling rates of taps P and Q:**
- Tap P can fill the tank in 15 hours. Therefore, the rate of tap P is:
\[
\text{Rate of P} = \frac{1}{15} \text{ tank per hour}
...