Abathtub is being filled at a rate of 2.5 gallons per minute. the bathtub will hold 20 gallons of water. how long will it take the bathtub to fill?

We know that the tub needs 20 gallons to fill, so our equation will be 

something something = 20

Now, we know that it fills at 2.5 gallons PER MINUTE. That per minute is very important because it tells us how many gallons are filled after a certain amount of time. 

So, we write the equation as 2.5x = 20. This is because our x (the unknown) is time, and it is filling up 2.5 gallons per time. So, divide 2.5 on each side, and we get 8 minutes as our answer.