Are you genius?

This is really amazing math problem! Five people can build five houses in five days! Trick is that you in few seconds answer next question! Only several people can give right answer! Open post, think few seconds and give answer!

Question is: How long would it take for 100 people to build 100 houses?

40

Answer is: 5 days because one man needs five days to build one house, so 100 people will need same time!