Seattle is $3$ hours behind New York. For example, when Seattle is $6$ AM, New York is $9$ AM. One day, Rosie took a flight from Seattle to New York. The flight took $$5$$ hours to reach New York, and the time was $9:40$ PM in New York when Rosie arrived. What was the time in Seattle when Rosie departed? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
It\textquotesingle s $9:40$ PM in New York, which means it\textquotesingle s $6:40$ PM in Seattle.  The plane departed at $6:40$-$5$ hours=$1:40$ PM. 