(12分之7减20分之9加30分之11减42分之13加56分之15)*24
=(1/3+1/4-1/4-1/5+1/5+1/6-1/6-1/7+1/7+1/8)*24
=(1/3+1/8)*24
=1/3*24+1/8*24
=8+3
=11