This is a geometric sequence with the first term 500, common ratio is (100 + 18)/100 = 1.18 and number of terms is 12.
Tn = ar^(n - 1)
T12 = 500 x 1.18^(12 - 1) = 500 x 1.18^11 = 500 x 6.176 = 3,087.96
Therefore, there is 3,088 fishes in the school in 12 years.
