The answer is the least common multiple of $4$ and $5$, which is $20$. 
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? 

[a] $$4$$ 
[b] $$5$$ 
[c] $$10$$ 
[d] $$20$$ 
[e] It depends on the distance around the track 