其它
Light Bulb A lights up every $$15$$ minutes while Light Bulb B lights up every $$20$$ minutes. Both light bulbs lit up at the same time at $$8.30$$ a.m.  By noon, how many more times would both light bulbs have lit up at the same time? 
Overseas Competition->Knowledge Point->Combinatorics->Time Problem
The LCM of $$15$$ and $$20$$ is $$60$$.  Both light bulbs will light up at the same time every $$60$$ minutes, at $$9.30$$ a.m., $$10.30$$ a.m. and $$11.30$$ a.m. (total of $$3$$ times). 