21 and 7/8-8 and 5/12
remember that 21 and 7/8=21+7/8 and 8 and 5/12=8+5/12 so
21+7/8-(8+5/12)
21+7/8-8-5/12
21-8+7/8-5/12
13+7/8-5/12
so 7/8 and 5/12
make bottom number the same
multiply 7/8 by 12/12 and multiply 5/12 by 8/8
7/8-5/12=84/96-40/96=(84-40)/96=44/96=11/24
so the answer is
13 and 11/24
