4 ms·
My favorite is still the multiples of 9 by putting down one finger. It amazes me. Of course it only works with a full set of fingers, but still.
by joshmn 11y ago
My favorite is still the multiples of 9 by putting down one finger. It amazes me.
Of course it only works with a full set of fingers, but still.
- venomsnake 11y agoMultiply by x by ten, substract x?
- booli 11y agoI was wondering why no one was suggesting this, isn't this the most obvious one?
- wingerlang 11y agoI always found 9 to be easy because whatever you multiply with (2-10) you just had to remove 1 from that number, and maybe memorise the second digit. e.g. 9*3 = 3-1 = 2 (+ the 7 that I just memorised) But actually the finger trick is amazing, and I just used it to double check the numbers as it was a loooong time I did it.
- kayge 11y agoThe "remove 1" trick is sort of how I learned it, except instead of memorizing "2 goes with 7" I learned that the 2 digits should add back up to 9: 9*3 = 27 (3-1 = 2,7 = 9-2) 9*4 = 36 (4-1 = 3,6 = 9-3) ... 9*9 = 81 (9-1 = 8,1 = 9-8) Fun stuff :)
- downtide 11y ago9s are easy if you know how: http://www.trachtenbergsystem.org/multiplication-9/ http://www.trachtenbergsystem.org/multiplication-9/
- jarcane 11y agoA fun property I learned that helps me remember the 9s: The digits of every multiple of 9, when totaled, eventually add up to nine. For 9 times x, for 2-10 at least, this can be reverse engineered: The 10s digit of the product will be x-1, and then the 1s digit will be whatever number remains that's needed for the digits to add up to 9. So 9 times 7: 7-1 = 6, 6+3 = 9, therefore 9*7 = 63. There may be a similar trick for bigger numbers, but I rarely need to do them quickly so I've never found one.