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/sci/ - Science & Math

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[ERROR] No.3608185 [Reply] [Original]

75*82 = ?

If you can't do this type of math in your head in less than 30 seconds, you should probably change your major to "exercise science."

>> No.3608197

Have a sage, little troll.

>> No.3608196

560+10+140+400 etc.

>> No.3608206

This reminds me of that part in the vidya game The Longest Journey, where you challenge an evil wizard to an arithmetic face-off. He doesn't know what a calculator is so he lets you use your "useless device" and he loses.

I was solving the problems in my head faster than the character could get the lines out.

Also, 6150

>> No.3608202

5664
if i'm wrong i will rage
used binomial expansion like a faggot

>> No.3608215

75 = 3 quarters.
82 x 3 = 246
246 quarters = 61.50 = 6150

>> No.3608217

<span class="math">75 \cdot 82[/spoiler]
<span class="math">75 \cdot 41 \cdot 2[/spoiler]
<span class="math">2(70+5)(40+1)[/spoiler]
<span class="math">2(2800 + 200 + 70 + 5)[/spoiler]
<span class="math">2 \cdot 3075[/spoiler]
<span class="math">6150[/spoiler]

>> No.3608224

6150

a good minute and a half, and I had to type it

>> No.3608240

75x2 = 150
75x80 = 6000
6000+150=6150

Come on guys that's 5th grade shit

>> No.3608252

6150
didn't cheat
did it my head in 45 seconds

lol @ butthurt fags ITT. It really is a sign of intelligence.

>> No.3608254

6150 all in my head without any "tricky" shit, simply
75
x 82

You are too kind for making the numbers work out so nicely.

>> No.3608258

(70 + 5)*(80 + 2) = 5600 + 400 + 140 + 10 = 6150

easiest way so far imo

>> No.3608266
File: 58 KB, 573x768, lol.png [View same] [iqdb] [saucenao] [google]
[ERROR]

>>3608240
>pic related

>>3608258
82 x 3 = 246
246 / 4 = 61.5
61.50 x 100 = 6150

>> No.3608282

70*80 + 70*2 + 80*5 * 2*5

anyone else do it this way?

>> No.3608305

>>3608282
>70*80 + 70*2 + 80*5 * 2*5
>80*5 * 2*5

No.

>> No.3608306

Here was my though process... damn this took a lot longer to type than it did to do mentally.

75 * 10 = 750
750 * 8 = 0.75 * 8 * 1000 = 6 * 1000 = 6000

75 * 2 = 150

6000+150=6150

>> No.3608316

who are you and how do you know my PIN number

>> No.3608323

6150 and i did it in less than 20 seconds
did it like this in my head:
   75*
   82
  150
 600
 6150

>> No.3608330

>>3608240
>>3608323
Anything else just seems like diving into unnecessary complications

>> No.3608333

My train of thoughts goes like this when asked to perform mental calculations

75 * 100 = 7500
75 * 10 = 750
7500 - 2*750 = 75 * 80
2 * 75 = 150
7500 - 2 * 750 + 2 * 75 = 6150

Or perhaps the less complicated way:
(70 + 5)(80 + 2) =
5600 + 140 + 400 + 10 = 6150

>> No.3608334

7 seconds.

82*75

2x75 = 150
80 x 5 = 400
150 + 400 = 550

70 x 80 = 5600

5600 + 550 = 6150

>> No.3608344

>>3608185

I prefer my Ph.D in Leisure Studies.

>> No.3608354

my thought process (did it in a few seconds) was this

75 = 100 * 3/4

3/4 * 82 = arithmetic_mean (82, 82/2=41) = 61.5

multiply by 100 to get 6150

of course, you should be able to do arbitrary 2 by 2 multiplications in well under 30s

>> No.3608355

>>3608215
This is how I imagined it. Whenever I see multiples of 25, I think in terms of money. 246 quarters was my thought exactly.

Other special situations worth mentioning:
<span class="math">49 x 51 = 50^2 - 1 = 2500 - 1 = 2499[/spoiler]
<span class="math">(n - 1)(n + 1) = n^2 - 1[/spoiler]

<span class="math">14 x 41 = 170 + 404 = 574[/spoiler]
<span class="math">(10a + b)(10b + a) = 10(a^2 + b^2) + 101ab[/spoiler]

>> No.3608356

My thought processes go like this:
75*82 =
80*75 + 2*75 =
10 * 8 * 75 + 2 * 75 =
10 * 4 * 150 + 2 * 75 =
10 * 2 * 300 + 2 * 75 =
10 * 600 + 2 * 75 =
6000 + 2 * 75 =
6000 + 150 =
6150

>> No.3608377
File: 928 KB, 822x687, joker-luthor-jazz-hands.1310065472167.png [View same] [iqdb] [saucenao] [google]
[ERROR]

(3*5*5)*82
410
2050
6150

What's 78*82?
6150 + (3*80 + 3*2)
.. 240 + 6
6396

>> No.3608391

The ability to do mental arithmetics is only loosely based on an individual's ability to do actual mathematics

>> No.3608401

i went all through high school doing everything in my head, when i got to math i couldn't do in my head in college i didn't know what the fuck to do and just gave up.

They were fucking right about developing the habit of writing it all out even if you can do it in your head.

Now i check even simple addition twice because im afraid of making mistakes.

>> No.3608412

6150 in like 15 or 20sec

150 + 80*70 + 80*5

I don't even bother keeping track of zeros, first I do 8*7 and 8*5 then I add zeros as needed.

>> No.3608457

>>3608356
I like that technique. Divide and conquer.

>> No.3608483

>>3608412
>I don't even bother keeping track of zeros, first I do 8*7 and 8*5 then I add zeros as needed.

I don't even bother keeping track of numbers, first I do 1+1 then I add 1 as needed.

6150

>> No.3609147

bump

>> No.3609205

intradesting how everyone has different approaches

75*2=150 plus 75*8*10, broke that into 150*4 then 300*2

i was never good at mental math but this problem worked out easy. i probably would have gave up if it didnt work out so nice with 6000+150

>> No.3611239

50*82=1/2 of 8200=4100
25*82=1/4 of 8200=2050

Together it's 6150, right?

>> No.3611247

25*4=100
100*2=200
200*10=2,000
2,000*3=6,000
6,000+150=6,150

That was my mental process I think. Solved it very quickly.

>> No.3611289
File: 89 KB, 445x445, 1307812258681.jpg [View same] [iqdb] [saucenao] [google]
[ERROR]

>2011
>Doing anything other than

>80 * 70 = 5600
>2 * 70 = 140
>80 * 5 = 400
>2 * 5 = +10
-----------------------
.............6150

ISHYGDDT
>

>> No.3611314

100*82 - 100*(82/4)
seemed the fastest way to me by far...
3 operations, like 5 seconds each maximum

>> No.3611332

about 20 seconds.
>close enough to 75*80
>easy, that's just 75*2*4*10=6000
>bring back that extra 2*75 and add it on
>6150

>> No.3611367

75*100=7500
75*20=1500
7500-1500=6000
6000+75*2=6150

>> No.3611376

75 * 10 = 750
750 * 2 = 1500
1500 * 4 = 6000
6000 + 75*2 = 6150

did it in less than 10 seconds. The weird thing is I don't even consciously know how my brain chooses to break down multiplication like that, it just happens naturally.

>> No.3611826
File: 161 KB, 600x600, 1224136034061.jpg [View same] [iqdb] [saucenao] [google]
[ERROR]

derp

>> No.3611863

7 * 8 = 56.
.5 * 8 = 4.

Put the zeroes back on, add together, you get 6000.

Then just times 2 by 75, get 150, and tack it on the end to get 6150.

>> No.3611884

I probably did this a weird way but whatever...

80^2 = 6400
78 * 82 = 6396

Subtract 3*82, or 246

6396 - 246 = 6150

Mathematics faggots, learn to do it

>> No.3611887

>>3611884

>Implying ability to add numbers up quickly is in ANY WAY correlated with the ability to be able to deal with highly abstract mathematical concepts

>> No.3611893

>>3611887
Butthurt faggot who's jelly of my adding-up-numbers skills detected

>> No.3611897

>>3611893

I managed to add them up in about 10 seconds; your assumptions are pathetic and you should probably just go to bed.

>> No.3611909

>>3611897
I manage to live in Australia, your assumptions about me living in America is pathetic and you should probably go kill yourself being behind us in time.

>> No.3611913

>>3611909

I live in Australia too: how fucking ironic it is that you assume I live in America.

>> No.3611930

>>3611913
Generally if you tell me to go to bed, then it would mean that it is sleepy weepy time wherever you live.
And since America is the arsehole of the world and you are clearly an arsehole, I put 2 and 2 together and posted a comment on an internet image board.

>> No.3611939

>>3611930

America, the asshole of the world? You mean the place that saved us from the Japanese in World War II? You mean the place that ensured that every new expensive movie is in English? You mean the place that safeguards Australia against Asian superpowers that want our resources?

Yeah, just kill yourself you little shit.

>> No.3611946
File: 29 KB, 631x460, 1310061472664.jpg [View same] [iqdb] [saucenao] [google]
[ERROR]

>>3608240
same thing i did. probably 12 seconds.

i love how people think OP is trolling cause they're literally too stupid to use multiplication without calculators/pen&paper

>> No.3611951

>>3611939

>assures*

>> No.3611956

>>3611939
>American butthurt fag who just found all those facts on Wikipedia 5 minutes ago pretending to be Australian detected

Lol, I have better things to do than argue with stupid faggots on the internet. Have fun fapping, crying and dying alone in your parent's basement you pretentious fuck.

>> No.3611963

>>3611956

Hahaha, oh god It's hilarious. You probably sit here all day on /sci/ receiving Centrelink benefits paid for by my taxes, and you have the nerve to champion how great this country is.

Give me a break you fucking loser.

>> No.3611967

70*80=5600
5*80=400
75*2=150
6150

>> No.3612120

75 x 100 - (75 x 8 + 75x 10)
20 seconds
could have done in 10-15 if i used some aforementioned strategies, my way usually works best though, as if it wasn't exactly 75 you guys woulda been fucked. I'm 15 btw

>> No.3612134

can't believe no one's mentioned this
75*4 = 300
80 = 4*20+2
300*20+75*2 = 6000+150 = 6150

>> No.3612135

>>3612134
*82 = 20*4+2 not 80

>> No.3612144

75*82
25*3*82
25*246
25*250-4*25
25*25*10-4*25
125*5*10-4*25
625*10-4*25
6250-100
6150
How i did it in my head pretty fast.

>> No.3612145

My thought process:

75*82
...
6150

you're all weak minded.

>> No.3612174

75
82
__
0150
6050
____+
6150

amidoingitright?

>> No.3612190

75 * 2 = 150
150 add a zero for it to be 20 times
1500 * 4 = 6000
6000 + 150 = 6150