规律:
16 =16
21 =21
16+7/8 =16+7/8
20+1/4 =21-6/8
17+3/4 =16+7/8+7/8
19+1/2 =21-6/8-6/8
接下来是:
16+7/8+7/8+7/8
21-6/8-6/8-6/8
.