albertlee Posted January 4, 2005 Share Posted January 4, 2005 It takes me 5 minutes to calculate: 1682 * 21 How fast can you calculate??? (without a calculator) Albert Link to comment Share on other sites More sharing options...
YT2095 Posted January 4, 2005 Share Posted January 4, 2005 do you mean in your head, or with a paper and pencil? Link to comment Share on other sites More sharing options...
albertlee Posted January 4, 2005 Author Share Posted January 4, 2005 well, for both!! what I did was with paper and pencil... For head, I did it in approx 15 minutes... Albert Link to comment Share on other sites More sharing options...
YT2095 Posted January 4, 2005 Share Posted January 4, 2005 just over 3 mins on paper, it was pointless doing it in my head after, coz I knew the answer edit, I`de have taken longer than you if I hadn`t used a few shortcuts though, I`m slow at maths. Link to comment Share on other sites More sharing options...
albertlee Posted January 4, 2005 Author Share Posted January 4, 2005 When guys started to learn time-table, even you just knew what is "muliplication" and to manipulate it, did you ever read the time-table out loud to memorize them???? Albert Link to comment Share on other sites More sharing options...
YT2095 Posted January 4, 2005 Share Posted January 4, 2005 I never did the outloud bit (not as I can remember anyway). but I DO rem waking up in the middle of the night with an Epiphany, I suddenly not only KNEW some of the times tables, but actualy understood WHY it was that way! it all made perfect sense, like seeing a pattern in one of the stereographs that you have to stare at for ages to make the picture out, until you learn the "secret" needless to say I jumped out of bed into my mom and dads room and proceeded to explain what happend (ad nausium). they obviously weren`t as exstatic as I was, but I was only 4, so it`s understandable in a way Link to comment Share on other sites More sharing options...
ydoaPs Posted January 4, 2005 Share Posted January 4, 2005 just over 2 min on paper. i am slow, maybe i should practice my calculation on paper. Link to comment Share on other sites More sharing options...
JaKiri Posted January 4, 2005 Share Posted January 4, 2005 Just under a couple of minutes (in head). Link to comment Share on other sites More sharing options...
fairychild Posted January 4, 2005 Share Posted January 4, 2005 i dont really see the challenge in using paper and pencil? Link to comment Share on other sites More sharing options...
YT2095 Posted January 4, 2005 Share Posted January 4, 2005 I do! )) Link to comment Share on other sites More sharing options...
ydoaPs Posted January 4, 2005 Share Posted January 4, 2005 i dont really see the challenge in using paper and pencil? good luck if you try to take the ASVAB. Link to comment Share on other sites More sharing options...
fairychild Posted January 4, 2005 Share Posted January 4, 2005 i think 6*8 looks to be the most difficult in there.. btw there is an easy way to calculate the square of a number ending with '5'. 145²: 14*14+14 = 210 (first digits) 25 (last digits, always the same) so: 21025 once i won a bet to calculate 1455² in my head within 10 minutes.. and won about 1 € Link to comment Share on other sites More sharing options...
YT2095 Posted January 4, 2005 Share Posted January 4, 2005 "168^2 * 21" I took 168 and multiplied it by 168, obtained the answer, we`ll call "result" then I took result and multiplied by 2, then added a 0 at the end then I added "Result" to that figure to get the final answer Link to comment Share on other sites More sharing options...
JaKiri Posted January 4, 2005 Share Posted January 4, 2005 14*14+14 = 210 (first digits) A more elegent (and less calculation intensive) way of doing it is 14*15. Similarly for other problems. This works for (x.5)^2 too; x*(x+1)+0.25 Link to comment Share on other sites More sharing options...
ydoaPs Posted January 4, 2005 Share Posted January 4, 2005 "168^2 * 21" I took 168 and multiplied it by 168' date=' obtained the answer, we`ll call "result" then I took result and multiplied by 2, then added a 0 at the end then I added "Result" to that figure to get the final answer [/quote'] that is exactly what i did Link to comment Share on other sites More sharing options...
Rasori Posted January 6, 2005 Share Posted January 6, 2005 YT's method, just under 2 mins pen + paper. And according to the calculator it even came out right Link to comment Share on other sites More sharing options...
albertlee Posted January 6, 2005 Author Share Posted January 6, 2005 14*14+14 = 210 (first digits) why add extra 14? Link to comment Share on other sites More sharing options...
ed84c Posted January 6, 2005 Share Posted January 6, 2005 I still think though it looks better, in the case of somebody asking this to you, To just sit therefore a couple of mins expressionless, in your head doing it quickly, and then blurt put the answer. Link to comment Share on other sites More sharing options...
fairychild Posted January 6, 2005 Share Posted January 6, 2005 why add extra 14? because the result would be wrong otherwise? or multiply with 15 as already mentioned Link to comment Share on other sites More sharing options...
JaKiri Posted January 6, 2005 Share Posted January 6, 2005 why add extra 14? 145 x 145 = (140 + 5) x (140 + 5) = 140 ^ 2 + 2 x 140 x 5 + 5 x 5 = 140 x 140 + 140 x 10 + 25 = 140 ( 140 + 10 ) + 25 = 140 x 150 + 25 Link to comment Share on other sites More sharing options...
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