Fun Mathy Type Riddles

[Haipule]

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

In vortex math there is no zero and no number higher than 9. Therefore, since 13 is higher than 9, it is reduced to 1 + 3 = 4. Therefore, 7 x 4 = 28 However, 28 is higher than 9 and is reduced to 2 + 8 = 10 which is also higher than 9 and reduced to 1 + 0 = 1. Conclusion: 7 x 13 = 1 or, shortcut 7 x 13 = 91 where 9 + 1 = 10 reduced to 1 + 0 = 1