其它
A school has two classes in the morning, each class continues $$45$$ minutes, and there is a break between classes. The first class starts at $$10:00$$, and the second class ends at $$12:00$$. How long does the courseware rest? 
Overseas Competition->Knowledge Point->Combinatorics->Time Problem
From $$10:00$$ to $$12:00$$ = $$2$$ h = $$120$$ min, two $$45$$-minute parts = $$90$$ min, and $$120 - 90 = 30$$ min. 