(15+15)/(1-5/60)
=30/(1-1/12)
=30/(11/12)
=30*(12/11)
=360/11
=32又(8/11)分
某时刻时钟和分钟正好成90度角,至少经过32又(8/11)分钟时间,时钟和分钟再次构成90度角.