(1/5)x(2/9)+(7/9)/5 =2/(5x9)+7/(5x9) =9/(5x9) =1/5 （12分之7减18分之5）除以12分之一减3分之2 =(7/12)x12-(5/18)x12-2/3 =7-10/3-2/3 =7-12/3 =7-4 =3
