It takes me 5 minutes to calculate:

 

168² * 21

 

How fast can you calculate??? (without a calculator)

 

Albert

do you mean in your head, or with a paper and pencil?

well, for both!!

 

what I did was with paper and pencil...

 

For head, I did it in approx 15 minutes...

 

Albert

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.

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

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

just over 2 min on paper. i am slow, maybe i should practice my calculation on paper.

Just under a couple of minutes (in head).

i dont really see the challenge in using paper and pencil?

I do! ))

i dont really see the challenge in using paper and pencil?

 

good luck if you try to take the ASVAB.

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 €

"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

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

"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

YT's method, just under 2 mins pen + paper. And according to the calculator it even came out right

14*14+14 = 210 (first digits)

 

why add extra 14?

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.

why add extra 14?

because the result would be wrong otherwise? or multiply with 15 as already mentioned

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