4620 = 2^2 * 11 * 3 * 5 * 7
20,21,22
20 = 2^2 * 5
21 = 3 * 7
22 = 2 * 11
20,21,22 的最小公倍数是 2^2 * 3 * 5 * 7 * 11 =4620