(20-50*(60/360))/(1-5/60)
=(20-50/6)/(1-1/12)
=(20-50/6)/(11/12)
=(20-50/6)*(12/11)
=(240-100)/11
=140/11
=12又(8/11)分
(20+50*(60/360))/(1-5/60)
=(20+50/6)/(1-1/12)
=(20+50/6)/(11/12)
=(20+50/6)*(12/11)
=(240+100)/11
=340/11
=30又(10/11)分
为下午的4时12又(8/11)分和4时30又(10/11)分.