**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.