We can let the variables A and B stand for the length in inches of fish A and fish B, respectively.
If we assume each person bought fish having a total length of 1 inch per gallon of aquarium, then we can write equations ...
... 3A +2B = 33 . . . . . total length of Marta's fish
... 3A +4B = 45 . . . . . total length of Hank's fish
Subtracting the first equation from the second, we get ...
... 2B = 12
... B = 6 . . . . . divide by 2
Using this value in the first equation, we have ...
... 3A + 2·6 = 33
... 3A = 21 . . . . . . . . subtract 12
... A = 7 . . . . . . . . . . divide by 3
Fish A is 7 inches long; fish B is 6 inches long.
