[BillTheCat]

A simple trick: For 11 x a 2-digit number you can simply take the first digit of the 2-digit number then the 2 digits of the number added together then the last digit of the 2-digit number.

Ex: 11 * 12 = 132 or 1, 1+2, 2. 11 * 45 = 495 or 4, 4+5, 5. For numbers which sum to more than 10 add the carry to the first number ex: 11 * 59 = 649 or 5, 5+9 = 14 so add 1 to the initial 5 and keep the 4, 9.

[gshubert17]

This generalizes to multiplying any number times 11,

https://en.wikipedia.org/wiki/Trachtenberg_system#Multiplyin...

Take a number, say 142857. Prepend a 0 on the left. Underneath each digit write the sum of the digit above and the one to the right. If you work from the right, it's easier to keep track of the carries.

  0142857
  1571427

[pc86]

I'm 29 and never heard this before. Neat!

[the-dude]

This is golden: I am just over 40 and I now know that I knew nothing at 29!