Since this question does not specify the depth of the pool it is unanswerable.

However, let us assume a depth of 5'.One gallon is equal to 0.134 cubic feet.
To determine how many gallons are in a 15' x 36' x 5' pool, you must first determine how many cubic feet are in the pool.

To do this, multiply length x width x depth.

15 x 36 x 5 = 2700 cubic feet

To determine the number of gallons in this pool, simply divide 2700 by 0.134

2700/0.134 = 20149 gallons.

1 gallon 20149 gallon

-------- = ------------

0.134 cf 2700 cf

Note that this calculation is for US gallons.