I'll start you off...

Define your variables: I'd call thebigger pump and thesmaller pump

the number of tanks per hour the bigger pump can empty

the number of tanks per hour the smaller pump can empty

Both of those numbers will be fractions.

Now, since you only wanted a hint, I'm only going to give you the first equation and you'll have to find the rest...

Two pumps of different sizes working together can empty a fuel tank is 5 hours.

Do you need any more help?