（1/4-1/15*2）÷2/5
=(1/4-2/15)x5/2
=1/4x5/2-2/15x5/2
=5/8-1/3
=7/24
4/7*（1÷3/4-3/4÷1）
=4/7x(4/3-3/4)
=4/7x7/12
=1/3