[1/2-(3/4-3/5)]÷7/10
=[1/2-3/4+3/5]×10/7
=1/2×10/7-3/4×10/7+3/5×10/7
=5/7-15/14+6/7
=5/7+6/7-15/14
=11/7-15/14
=1/2