25分之18减23分之9的差除以25分之9减23分之21
(18/25-9/23)÷（9/25-21/23）
=[(18*23-9*25)/(25*23)]
÷[(9*23-21*25)/(25*23)]
=9(46-25)/[3(69-175)]
=3*21/(-106)
=-63/106.