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/sci/ - Science & Math

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12130904 No.12130904 [Reply] [Original]

How do you do 3.5 * 2.4 in your head

>> No.12130926

3.5*2=7
3.5*4=14, so 3.5*0.4 is 1.4

3.5*2 4 = 3.5*(2+0.4) = 3.5*2 + 3.5*0.4 = 8.4

>> No.12130940

>>12130904
(35*2*10+35*4)/100

>> No.12130945

3.5=7/2
2.4=12/5
7*12/2*5=84/10=8.4

>> No.12130950

>>12130904
3.5*2 = 7

3.5*.4 = 3.5*.2*2 = .7*2 = 1.4

8.4

>> No.12130956

3*2.4 + 0.5*2.4 = 7.2 + 1.2 = 8.4

>> No.12130965

3*2+3*0.4+0.5*2+0.5*0.4
=6+1.2+1+0.2
=8.4

>> No.12130967

>>12130904
3*2.4 + 1.2
6+1.2 + 1.2
8.4

>> No.12130975

>>12130904
3.5 * 2.4 = 7 * 1.2 = 8.4

>> No.12131478

>>12130904
3*24=72 so 30*24 = 720
then past freshman year you probably know your factorials so you immediately recognize 5*24=5!=120 to obtain 720+120 = 840
ergo 3.5*2.4 = 8.4

>> No.12131516

>>12130904
35 x 2 = 70(0)
35 x 4 = 140
700 + 140 = 840
8.4

>> No.12131569

>>12130904
3*2+3*0.4+0.5*2+0.5*0.4=6+1.2+1+0.2=8.4

>> No.12131582

3.5*2.4 approx = 4*2 = 8

>> No.12131595

>>12130904
I don't. I just use a calculator.

>> No.12131605

>>12131582
This

Any other answers are a waste of time and have no real world application

>> No.12131614

>>12131605
>Any other answers are a waste of time
It should only take about 10 seconds to get the exact answer. Estimating could be disastrous in a number of real world scenarios.

>> No.12131628
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12131628

>>12130904
How do you do [math]\pi^e*e^\pi[/math] in your head?

>> No.12131638

>>12131614
>t. theoretical physicist

>> No.12131649

2.4 * 10 = 24
24 * 3 = 72
24 * 0.5 = 24 / 2 = 12
72 + 12 = 84
84 / 10 = 8.4

>> No.12131704

>>12131628
3^4*4^3 = 9*9*16*4 approx = 10*10*20*5 = 519.72046555170510160745260067332853596839220140738214774032038924

>> No.12132154

>>12130904
Easiest way is 3*2.4=7.2 , .5*2.4=1.2, 7.2+1.2=8.4

>> No.12132180

>>12130904
probably
35 x 24 = (35 x 25) - 35 = 25x25 + 25x10 - 35 = 840
Then move the decimal.

>> No.12132266

~3 * ~3 = ~9

>> No.12132272

>>12130904
35*20=700
35*4=140
700+140=840
840/100=8.4

>> No.12132279

>>12130904
35*24 = 35*20 + 35*4 = 700 + 140 = 840
Now move decimal two places: 8.4

>> No.12132676
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12132676

>>12131595
>I don't. I just use a calculator.

>> No.12132694
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12132694

>>12130904
.4 * 3.5 = 1.45
2 * 3.5 = 7
7 + 1.45 = 8.45

>> No.12132696
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12132696

>>12132694
Fuck, I was off by .5

>> No.12132697

>>12131628
3^6 = 729

>> No.12132727

>>12130904
(70 * 20 + 70* 4) / 200

>> No.12132735

>>12130904
2*7-5*2+6*4-29+10-6/10=14-10+24-29+10-.6=28-29+10-.6=9-.6=8.4

>> No.12133022

24 = 4!
35 = 7*5

35*24 = ((7*3)*4) * (5*2)
= (21*4) * 10
= 840 move 2 decimal places

8.4

>> No.12133032

24= 4!
35= 7*5
35*24= 7*5!

>> No.12133098
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12133098

>>12130904
>3 point 5 times 2 point 4 that's uh
>3 times 2 is 6 so it's close to uhh
>let me get my calculator

>> No.12134834

>>12130904
3x2 + .4x.5
Simple as

>> No.12134839

>>12134834
That's wrong answer anon

>> No.12134854

>>12130904
3.5*2.4
= (7*2.4)/2
= (7*2+7*0.4)/2
= (14+7*4/10)/2
= (14+2.8)/2
= 16.8/2
= 8.4
I would probably use a calculator though

>> No.12134968

3,5*2=7
3,5/2=1.75
3,5/10=0,35
1,75-0,35=1,4
7+1,4=8,4

>> No.12134971

maybe like around 9 or 10

>> No.12135118

3.5*4+3.5*20=8.4

>> No.12135132

>>12130904
3.5*2.4
35*24
35*25
35*100
3500
3500/2=1750
1750/2=875
875-35=840
840*.01=8.4

>> No.12135227

I dreamt that the answer is 8.4

>> No.12135242

>>12130904
3.5*2.4
3.5*2.4*10*10
35*24
35*2*10+4*35
840
840/10*10
840/100
8.4

>> No.12135364

>>12130904
3*2.4 = 6.(3*.4)
3*.4 = 1.2
7.2
0.5*2.4 = 1.2 (huh, same number)
7.2+1.2 = 8.4

>> No.12135472

>>12130904
2*35=70
20*35=700

35*2*2=
70*2=140
700+140=840
840/100=8.4

>> No.12135509

>>12130904
7 + 0.7*2 = 8.4
Second summand goes from the fact that 0.4 is 2/5 and 1/5 of 3.5 is 0.7

>> No.12135578

>>12130904
Immediately recognising that one has a decimal part of 0.5, I double the part containing 3.5 and halve the other. I then separate the resultant 7*1.2 into 7*1+7*0.1+7*0.1 and add them in my head.
I am employed as an accountant for a hospital network.

>> No.12135583

>>12135578
No wonder health care is so expensive.

>> No.12135620

>>12130904
First i multiply 3.5 by 40%
to do this i take 35 and multiply it by 4
to do this i take 4 times 5 and thats 20
then 30 and 30 is 60, which means 120+20,which is 140
i now know that 40% of 3.5 is 1.4

I then add 1.4 to 3.5 times 2, which is 7 because 3 times 2 is 6 plus 1. so i get 8.4.

>> No.12137184

>>12130904
I write it out in my imaginary note pad

>> No.12137216

>>12130904
3 * 2 = 6
0.5 * 0.4 = 2

6 + 2 = 8

>> No.12137225

>>12130926
Holy shit thank you

>> No.12137230

>>12130904
3.5*2=7
3.5/10=0.35
0.35*4=1.4
1.4+7=8.4

>> No.12137232

3.5 * 2 = 7
3.5 * .4 = 3.5 * 4/10 = 3.5 * 2/5 = .7*2 = 1.4
7 + 1.4 = 8.14

>> No.12137378

I dots

..................................................................................../..........

>> No.12137609

>>12130904
1.2+7.2

>> No.12137631

>>12130904
3*2 6
*.4 1.2
7.2
.5 * 2.4 1.2
8.4

>> No.12137706

>>12132676
based engineer

>> No.12137877

>>12132696
anon...

>> No.12138424

>>12130904
Do 35*25 - 35 and move the decimal, two places to the left.

So lets start by doing 30 * 25. That's the same as 3 * 25 and then add a 0. So 750.

Lets now do 5 * 25. Notice that I bumped up 24 to 25 because it's like counting quarters. 4 quaters is 100 cents and adding another is 125 cents. So 125.

Now lets add 750 to 125. We've got a total of 8 hundreds (7+1) and 50+25 is 75 so : 875.

Finally, lets subtract 35 to bring us back to 35*24 instead of 35*25.

Since 75 is greater then 35, we know that the hundreds will stay the same, so we won't worry about it. Now, lets take 3 from 7 = 4. and lets take 5 from 5 = 9. The total being 840.

Lets not forget to move that decimal place to bring us to 8.40

That's what happens in my head, albeit a lot faster, when I do mental math.

>> No.12138564

>>12130904
35 * 3 * 2 * 2 * 2 /.100

>> No.12138567

Like Rachel Riley, by memorising double digit multiplications

>> No.12138680

>>12130904
>3.5*2.4
>(3+0.5)(2+0.4)
>6+1+1.2+0.2
>8.4

Slower than some other methods here. I find this to be the easiest way, personally.

>> No.12138705

>>12138424
(7 * 5) * (6 * 4) * 10^(-2)
This should be a memorized solution for numbers less than a hundred because it has a partial factorial form [n!/(n-k)!]
Anyways, if you are so impotent that you have to resort to regrouping, organize terms:
(7*6)(5*4) = 42*20 = 840
840 * 10^(-2) = 8.4
Just in case you were wondering, I am not belittling you. I am just leading the horse to a shallow grave.

>> No.12139096

>>12130904
3.5*2.4
= (35 * 24) / 100
= ((2 * 350) + 140) / 100
= 840 / 100
= 8.4
In about 3-8 seconds, depending on how awake I am.

>> No.12139452

>>12137184
I do this

>> No.12139498

>>12130904
3 x 2 = 6
3 x .4 =1.2
6 + 1.2 = 7.2
2.4 / 2 = 1.2
7.2 + 1.2 = 8.4

>> No.12139502

>>12130904
> 3 * 2.4 = 7.2
> 0.5 * 2.4 = 1.2
> 1.2 + 7.2 = 8.4

>> No.12139525

Olol

>> No.12139544

3.5 x 2 = 7
3.5 x .1 = .35
.35 x 4 = 1.4
7 + 1.4 = 8.4

>> No.12139576
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12139576

>>12130904
Here

>> No.12139629

3x2.4
+0.5x2.4

>> No.12139633

>>12139629
oh crap that went way wrong

>> No.12140295

>>12130904
(3.5*2) + (0.35*4)

>> No.12140300

>>12130904
3(2.4) +.5(2.4)
7.2 + 1.2

>> No.12140406

3.5*2=7
30*4+5*4=140
140/100=1.4
7+1.4=8.4
I'm dumb

>> No.12142081

>>12130904

3*2.4 = 7.2

0.5*2.4 = 1.2

1.2 + 7.2 = 8.4

>> No.12142095

35 * 4
2 * 350
140 + 700
840 / 100
Return 8.4

>> No.12142153

>>12130904
2.4 x 3 = (6 + 1.2 = 7.2) then
2.4 x 0.5 = 1.2 then
1.2 + 7.2 = 8.4
Sometimes I try it the other way to see if it’s faster, but 40% of 3.5 is more difficult to just immediately think of as opposed to 50% of 2.4

>> No.12142862

>>12139502
My nigga

>> No.12142982

>>12130904
I don’t.
.t math major