如果a除以3,5,7,余1,则有a－1是3,5,7的公倍数,3,5,7在2000内的公倍数有105,210,315,420,525,630,735,840,945,1050,1155,1260,1365,1470,1575,1680,1785,1890,1995,再加上0
就是105乘1,105乘2……
a-1有了,20个,a也20个,就像上面说的那样.