(3.1/15-1.1/15:1.3/5+2/5)* 2.1/7-1.1/7=
1)1.1/15:1.3/5=16/15:8/5=16/15*5/8=2
2,3)3.1/15-2+2/5=1.(1+6)/15=1.7/15
4)1.7/15*2.1/7=22/15*15/7=22/7=3.1/7
5)3.1/7-1.1/7=2