Chỉ bài 2 thôi nha mọi người

1, `30/24=15/12`

2, `3/21=1/7`

3, `9/24=3/8`

4, `6/9=2/3`

5, `4/6=2/3`

6, `8/10=4/5`

7, `45/25=9/5`

8, `36/24=3/2`

9, `40/55=8/11`

10, `28/40=14/20=7/10`

11, `15/45=3/9`

12, `56/70=8/10=4/5`

13, `18/90=2/10=1/5`

14, `30/36=10/12=5/6`

15, `32/72=16/36=8/18=4/9`

16, `48/54=16/18=8/9`

17, `5/25=1/5`

18, `35/75=7/15`

19, `240/160=24/16=3/2`

20, `21/630=7/210=1/30`

21, `42/180=14/90`