1/5+2/9÷5/9
=1/5+(2/9)*(9/5)
=1/5+2/5
=3/5
4/15*5/6*5/12
=(4/15)*(5/6)*(5/12)
=(2/9)*(5/12)
=5/54
4÷2/3÷9/20
=4*(3/2)*(20/9)
=40/3