7/10-2/7÷5/7
=0.7-2/7*7/5
=0.7-2/5
=0.7-0.4
=0.3
5/8*1/4+1/4*3/8
=1/4*(5/8+3/8)
=1/4*1
=1/4
（1/2-1/3）÷5/6+1/3
=1/6*6/5+1/3
=1/5+1/3
=8/15
4/5*（5/6-3/4）-1/15
=4/5*5/6-4/5*3/4-1/15
=4/6-3/5-1/15
=10/15-9/15-1/15
=0