57*7/56
=（56+1）*7/56
=56*7/56+1*7/56
=7+7/56
=7又56分之7
2/5*4/7+2/7*3/5
=4/5*2/7+2/7*3/5
=2/7*(4/5+3/5)
=2/7*7/5
=5 分之2