12分之5+20分之5+30分之5+42分之5+56分之5+72分之5+90分之5
=5×(1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10)
=5×(1/3-1/10)
=5×7/30
=7/6