To find out how many feet per hour the water level in the swimming pool decreased, we need to determine the rate of decrease given that the pool was emptied in 3 hours.

The pool is 15 feet deep, and it was drained completely over 3 hours. The rate of decrease can be calculated by dividing the total depth by the time taken:

$\text{Rate of decrease} = \frac{\text{Total depth}}{\text{Time}}$

Substitute the given values:

$\text{Rate of decrease} = \frac{15 \, \text{ft}}{3 \, \text{hours}}$

Now, perform the division:

$\text{Rate of decrease} = 5 \, \text{feet per hour}$

So, the integer representing how many feet per hour the water decreased is 5 feet per hour.

Would you like more details or have any questions?

Here are some related questions:

1. What would be the rate if the pool was drained in 5 hours?
2. How long would it take to drain a pool twice as deep at the same rate?
3. What is the volume of water drained if the pool has a surface area of 200 square feet?
4. How can we express this draining rate in inches per minute?
5. How do different drainage rates affect pool maintenance?

Tip: To find the rate of change in a situation like this, divide the total change by the time taken.