Fun Mathy Type Riddles

[SaStrike]

I came across one that may prove a right old poser:

It is a commonly accepted, but erroneous, belief that 7 x 13 = 91.  In fact, 7 x 13 = 28.  The can be checked by demonstrating that 28 / 7 = 13, and by showing that 13+13+13+13+13+13+13 = 28.

Hint:  Colgate Toothpaste

After a reasonable time for those who wish to have a go have had one, I'll post the solution.


Boru

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You sure you're not mistaking the number 13 for the number 4?

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How about we go for a classic mathy type riddle?

Suppose you're on a game show, and you're given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 3, and the host, who knows what is behind the doors, opens another door, say No. 1, which has a goat. He then says to you, "Do you want to pick door No. 2?"

Is it to your advantage to switch your choice?

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You have a 1/3 chance of getting the car. So that means 2/3 chance of NOT getting the car.

If the host shows you a door with a goat (keeping in mind that there is a 2/3 chance you did not get the car, which means a 2/3 chance that changing the door is the car.

That's how I understand this classic problem which if not thought is put into it can make it seem like a 50/50.

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