I suppose many might have figured this out, but it's a neat little mind trick for calculating the product of a number between 10 - 19 with itself. I stumbled upon it as I was riding the bus back home from my math class, and thought it might come in handy for those who don't know it yet!

Multiplication:

Think:

Excellent, should take a normally gifted person about seconds!

For numbers which gives two digits for any of the operations, you do the same basically. But just remember that you need to summarize the results you get correctly.

Multiplication:

Think:

If you remember the addition you need to do, it shouldn't take much longer.

This metod works excellently for myself, and it will usually take me between 2-5 seconds to calculate the product of any number between 10-19 multiplicated with itself.

A bit more mathematically we can write:

Where alpha denotes the first digit of the product, and beta the second.