5 ms·
Q - how do you multiply something like 25x63 example in your head? Personally, I'd break that down also, by doing - 5 x 63 = 315 x 5 = 1575 reminds me a littl
by rollthehard6 7y ago
Q - how do you multiply something like 25x63 example in your head? Personally, I'd break that down also, by doing -
5 x 63 = 315 x 5 = 1575
reminds me a little of the Feynman story about his duel with the abacus master. https://news.ycombinator.com/item?id=5849665 https://news.ycombinator.com/item?id=5849665
- docdeek 7y agoFor that example I would be thinking 60 x 25 = 1500 (4 x 25=100, so that's the simple part for me) and then 3 x 25 = 75, and add the two products.
- soVeryTired 7y agoIn general I find mental arithmetic like this easiest if you start from the most significant digits. So you just need to keep track of a running sum and which multiplications you still need to add. Here's the calculation as I would do it in my head, more or less: 25 * 63 = (20 + 5) * (60 * 3) = (20 * 60) + (5 * 60) + (3 * 20) + (5 * 3) = 1200 + (5 * 60) + (3 * 20) + (5 * 3) = 1500 + (3 * 20) + (5 * 3) = 1560 + (5 * 3) = 1575
- flashgordon 7y agoIf division can be involved, why not: = ((25 * 4) * 63) /4 = 100 * 63 /4 = 6300 / 4 = (6400 - 100) / 4 = 1600 - 25 = 1575