Given :
A bathtub holds 80 gallons of water.
Water drains out of the tub at a rate of 2 gallons every thirty seconds.
To Find :
How many minutes will it take a full bathtub to drain completely.
Solution :
We know, 30 seconds = 0.5 minute.
Rate of water flow per minute is given by :

Let, bathtub will completely drain in x minutes.
So, 4x = 80
x = 80/4
x = 20 minutes
Hence, this is the required solution.