Solution: 39
Surface area is directly proportional to the combined length of all fish) (given in question)
First Area = 16 * 18
Therefore, 16* 18 = k * 24 (where k is the constant of proportionality)
Also, for the second case
36 * 13 = k * Combined Length
Therefore, Combined Length = (36 * 13) / k
= (36 * 13 * 24) / (16 * 18)
= 39.
Hence, the tank can support 39 inches of rainbow fish.