What is the answer and how do I find it?

Question

The time that it takes to fill a fish tank depends upon the rate at which the water is flowing. It takes 40 minutes to fill the fish tank at the rate of 3 gallons per minute. How many minutes will it take to fill the fish tank at the rate of 4 per minute?

(Please show your work!)

Answer 1

This could be done with proportions

(40)(3) = (t)(4)

Answer 2

40x3=120 gal

120/4=30 min

It will take 30 minutes to fill the tank at 4 gallons per minute,