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.