(20-15)/(1-5/60)=5/(1-1/12)=5/(11/12)=5*(12/11)=60/11=5又(5/11)分 16:30时候是四十五度
(20+15)/(1-5/60)=35/(1-1/12)=35/(11/12)=35*(12/11)=420/11=38又(2/ 11)分
20/(1-5/60)=20/(1-1/12)=20/(11/12)=20*(12/11)=240/11=21又(9/11)分
(20+30)/(1-5/60)=50/(1-1/12)=50/(11/12)=50*(12/11)=600/11=54又(6/11)分
4时到5时分针与时针在四点5又(5/11)分和38又(2/11)分时互相垂直.
分针与时针在四点21又(9/11)分时重合