It took 5 1/4 gallons of gas to fill the cars gas tank. This is 3/8 of the amount the tank can hold. How much gas does the tank hold?

It took 5 1/4 gallons of gas to fill the tank. This is 3/8 of the amount the tank will hold. How much gas does the tank hold?

You can take it like this.

3 parts of the tank holds 5 1/4 gallons.
How much does 8 parts hold? (8 representing the total)

It's similar to:
3 apples cost $15
How much do 10 apples cost?
You look for one apple first, that is 15/3 = $5
Then, you say, 10 apples cost 5x10 = $50

Does it help?

EDIT: I had put 'gallon' instead of 'tank' thanks KISS.

Unknown:

3 parts of the gallon (Change gallon to tank) holds 5 1/4 gallons.