（2/15+1/12)×3+3/5
= 2/5 +1/4 + 3/5
=2/5+ 3/5 +1/4
= 1+ 1/4
= 5/4
4/9×【3/4-（7/16-1/4）】 简便运算
= 4/9 x 【3/4+1/4-7/16】
= 4/9 x (1-7/16)
= 4/9 x (9/16)
= 1/ 4