l/24x24x(7/12十5/8一l/2)=l/24X(14十15一|2)=|7/24
7/12+5/8-1/2=7/12+5/8-6/12=7/12-6/12+5/8=1/12+5/8=2/24+15/24=17/24
14/24+15/24-12/24=17/24