here is a math puzzle (not made by myself)
rules:
noone who has already seen the solution
no internet
solutions must be explained
A guy has robots with 7, 8 and 9 arms. Unfortunately, the ones with 7 arms a bugged and everything they say is a lie. The others are ok, they always tell the truth. One day the guy overhears 4 robots talking but he doesn't see them. They're talking about their arms. The first robot claims "Alltogether we have 28 arms."
"No" the second one said "we have 27!"
"26" said the third one.
And the fourth said "Wrong, only 25."
How many arms do they all have? (unknown is a possible answer as well for some)
have fun