How long will it take for the pool to completely full with water? (explain why so i understand)

First, you find out how much can be filled in 1 minute. Then you divide that into 120 to get your answer. 
To find out how much can be filled in 1 minute, take the time and divide it into the volume. We'll use the second row (2 minutes and 8 gallons.)
8/2=4. So it takes 1 minute to fill 4 gallons. Now, you just divide 4 into 120. 
120/4=30
It takes 30 minutes to fill the pool up to 120 gallons.