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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
by BillTheCat 11y ago
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.
- pc86 11y agoI'm 29 and never heard this before. Neat!
- deleted 11y ago[deleted]
- the-dude 11y agoThis is golden: I am just over 40 and I now know that I knew nothing at 29!
- gshubert17 11y agoThis generalizes to multiplying any number times 11, https://en.wikipedia.org/wiki/Trachtenberg_system#Multiplying_by_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