(21 + 3)÷2 = 12
12×18 = 216,答案是216.
因为：
(1 + 3)÷2 = 2,2×18 = 36
(5 + 3)÷2 = 4,4×18 = 72
(17 + 3)÷2 = 10,10×18 = 180