8/9-8/9×0.1
=8/9×（1-0.1）
=8/9×0.9
=0.8
4/5-0.8×0.1
=4/5×（1-0.1）
=4/5×0.9
=0.72
0.17×5/8×20
=0.17×20×5/8
=3.4×5/8
=17/8