Do you know about those cheat checks?

Tweet

I was clicking at one PTC and just came through a cheat check....whenever i see them i just keep wondering that how these simple maths 10+10=20 ...or....20/5=4...these problems can check whether we are cheat or not??..i can't understand what is the sense in that...and how that can confirm that we are not cheating?? can anyone clear my doubt?