а) 2 5/6-(1 5/9+1/4)=1 1/36
1)1 5/9+1/4=14/9+1/4=(56+9)/36=65/36
2)2 5/6-65/36=17/6-65/36=(102-65)/36=37/36=1 1/36
------------------
б) 7/20-(3 5/8-3 2/5)=1/8
1)3 5/8-3 2/5=3 25/40-3 16/40=9/40
2)7/20-9/40=14/40-9/40=5/40=1/8
-------------------------
в) 3/4-(4/25+7/20)=6/25
1)4/25+7/20=16/100+35/100=51/100
2)3/4-51/100=75/100-51/100=24/100=6/25