(40-15)/(1-5/60)
=25/(1-1/12)
=25/(11/12)
=25*(12/11)
=300/11
=27又(3/11)分
(40+15)/(1-5/60)
=55/(1-1/12)
=55/(11/12)
=55*(12/11)
=550/11
=60分
在8點27又(3/11)分9點整時,時針與分針成90°角.
(40-30)/(1-5/60)
=10/(1-1/12)
=10/(11/12)
=10*(12/11)
=120/11
=10又(10/11)分
在8點10又(10/11)分時,時針與分針在一條線上.