Teacher Anderson has pens with three different colours, $$12$$ black, $$8$$ red and $$10$$ green. He put all of these pens in his pencil case. If he close his eyes and randomly pick a pen, at least how many pen he must pick to guarantee he get a red pen?． 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
To guarantee he get a red pen, the worse cases scenario is he picked all the green and black pens. Which is $$12+10=22$$. Then, the next pen must be the red. Thus, $$22+1=23$$. 