15/16÷20×4/5
=15/16×1/20×4/5
=3/80
5/6+2/3÷8/9
=5/6+2/3×9/8
=5/6+3/4
=19/12
（7/8-21/32）÷7/8
=（28/32-21/32）×8/7
=7/32×8/7
=1/4