15.9/7 - （ 2/7 – 10/21 ）
= 9/7 - 2/7 + 10/21
=1+10/21
=31/21
5/9 × 18 – 14 × 2/7
=10-4
=6
4/5 × 25/16 + 2/3 × 3/4
=5/4+2/4
=7/4
14 × 8/7 – 5/6 × 12/15
=16-2/3
=15又1/3
17/32 – 3/4 × 9/24
=51/96-27/96
=24/96
=1/4