1) 2/9*(3/10*7/8)=2/9*3/10*7/8=
=(2/9*3/10)*7/8=1/15*7/8=7/120
2) 15/19*(19/25*7/9)*5/7= 15/19*19/25*7/9*5/7=
=(15/19*19/25)*(7/9*5/7)=3/5*5/9=1/3
3) (10/11*11/12)*(12/55*13/20)=10/11*11/12*12/55*13/20=
=(11/12*12/55)*(10/11*13/20)=1/5*13/22=13/110
