其它
A school has three classes in the morning, and there is a $$15$$-minutes break between each two classes. Each class is of the same length. The first class starts at $$9:45 $$, and the third class ends at $$12:00 $$. How long is each class? 
Overseas Competition->Knowledge Point->Combinatorics->Time Problem
From $$9:45$$ to $$12:00$$ = $$2$$ h $$15$$m = $$135$$ min, two $$15$$-minute parts = $$30$$ min, and $$135 - 30 = 105$$ min. $$105\div3=35$$min 