Question 912488
There are 2 ways to do this problem.
1. Cross multiplication 
             OR
2. Equivalent fractions

So let's do Method 1 first.

To do cross multiplication, you multiply the 2 known values diagonal from each other, then divide it by the remaining known value. This method is most useful when when trying to determine unknown values in fractions. 

So the question would look like this. 4/5=24/x
Then, you would make a that looks like this: x=5*24 divided by 4 
                                             x=30
Method 2
4*6=24
so
5*6=30

Therefore, when full, the gas tank can hold 30 gallons.