There are $$40$$ guests queueing to enter a party. Every $$4^{\rm th}$$ guest in the queue receives a balloon and every $$6^{\rm th}$$ guest in the queue receives a mask. How many guests receive both a balloon and a mask?~\uline{~~~~~~~~~~}~ 
Overseas Competition->Knowledge Point->Word Problem Modules->Distribution Problems->Basic Distribution Problems
Every 12th guest receives both a mask and a balloon. Hence, the 12th, 24th and 36th guest receive both. 