а) ......=20/21 - 16/21=4/21
4/21 * 100/67=400/1407
б) .....=3/4*7/3=7/4
7/4 : 2=7/4*1/2=7/8
7/8 * 100/28=7/8 * 25/7=25/7=3 4/7
в) ....=3 5/12 - (2 4/18 - 3/18)=3 5/12 - 2 1/18=3 15/36 - 2 2/36=1 13/36
1 13/36 * 54/100=49/36 * 54/100=(49*3)/(2*100)=147/200
47/200 * 100/21=47/42=1 5/42