6分之5减12分之7加20分之9减30分之11加42分之13巧算
问题描述：
6分之5减12分之7加20分之9减30分之11加42分之13巧算
答
原式等于：(5/6-7/12)+(9/20-11/30)+13/42
=(10-7)/12+(27-22)/60+13/42
=3/12+5/60+13/42
=3/12+1/12+13/42
=4/12+13/42
=4X7/84+2X13/84
=28/84+26/84
=54/84
=9/14.