If it takes you $$13$$ minutes to wash up in the morning, $$23$$ minutes to have your breakfast, $$8$$ minutes to pack the school bad and $$27$$ minutes to get from home to the school. The school rules require you to arrive at $$7.45 \rm{a.m.}$$ every morning, what\textquotesingle s the latest time you need to wake up? 
Overseas Competition->Knowledge Point->Combinatorics->Time Problem