Find the volume of the bath tub by multiplying all the dimensions together:
18 inches = 1.5 feet:
5 ft x 3 feet x 1.5 feet = 22.5 cubic feet.
The tub is 3/4 full so multiply the volume by 3/4 to find the amount of water in the tub:
22.5 x 3/4 = 16.875 cubic feet of water.
The tub drains at 1 cubic foot per minute, so it would take about 17 minutes to drain. (16.875 rounded to the nearest whole minute).