其它
Amy runs a lap around the track in $4$ minutes and Pawel in $5$ minutes. Amy and Pawel start to run around the track at the same time. After how many minutes will the boys meet at the starting point again? 
Overseas Competition->Knowledge Point->Number Theory Modules->Factors and Multiples
The answer is the least common multiple of $4$ and $5$, which is $20$. 