Define your variables: I'd call the bigger pump and the smaller 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?