(3/10-2/3*9/40)÷(3/5+9/10)
=(3/10-3/20 )÷(3/5+9/10)
=(3/10-3/20 )÷[(3/10+9/20)×2]
=(3/10-3/20 )÷(3/10+9/20)÷2
=1÷2
=1/2