First find the volume of the pool. To do so, multiply the length, times the width, times the height. In this case, it would be 12*15*5 and you get 900 cubic meters.
If the pool is being filled with 36 cubic meters of water per hour, you divide 900 by 36. You get 25 hours.
To check your work, multiply 25 hours and 36 cubic meters of water. You get 900 cubic meters, which is the area of the pool meaning it is full.
