0.3/5/6+5/2
=5/6*3/10+5/2
=4+5/2
=5/22
3/7/21*7/9
=7/21*3/1 *7/9
=7/9
12/5/（1-9/4）
=12/5 *5/9
=4/3
【1-（8/1+0.25）】/12/5
= 【1-（8/1+4/1）】*5/12
=5/12-10/3-5/3
=5/9-10/3
=2/3