How fast can you calculate??

[albertlee]

It takes me 5 minutes to calculate:

 

168² * 21

 

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

 

Albert

[YT2095]

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

[albertlee]

well, for both!!

 

what I did was with paper and pencil...

 

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

 

Albert

[YT2095]

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.

[albertlee]

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

[YT2095]

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

[ydoaPs]

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

[JaKiri]

Just under a couple of minutes (in head).

[fairychild]

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

[YT2095]

I do! ))

[ydoaPs]

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

 

good luck if you try to take the ASVAB.

[fairychild]

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 €

[YT2095]

"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

[JaKiri]

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

[ydoaPs]

"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

[Rasori]

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

[albertlee]

14*14+14 = 210 (first digits)

 

why add extra 14?

[ed84c]

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.

[fairychild]

why add extra 14?

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

[JaKiri]

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