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How to git gud at mental math and basic arithmetic?

Anonymous Tue 07 Jan 04:49:16 2020 No.11283659 View Reply Original Report

Quoted By: >>11283667 >>11284397 >>11285998 >>11286191 >>11286286 >>11286320 >>11287245 >>11287495

How do you solve something like 12*14 in your head? Do you do:
>10*14 + 2*14 =168
or do you have some other way of doing quick maths. I just realized after all this time I've been so dependent on a calculator that I've almost forgotten how to do basic arithmetic without one

Anonymous

Anonymous Tue 07 Jan 2020 04:51:30 No.11283667 Report

Quoted By:

>>11283659
i went...

12 to 14 12 to 14 12o to 14.. shit am i too slow? ... 12 to 14 ah 12 x12 is 144 cuz memory then 2x12 more is just 24 so 168 sum

Anonymous

Anonymous Tue 07 Jan 2020 08:43:38 No.11284093 Report

Quoted By:

14×2=28

14×10=140

140+28=168

Anonymous

Anonymous Tue 07 Jan 2020 10:12:19 No.11284216 Report

Quoted By:

Split up the numbers.
12x14 = (10+2)x(10+4)=10x10 + 2x10 + 4x10 + 2x4 = 100 + 20 + 40 + 8 = 168

Anonymous

Anonymous Tue 07 Jan 2020 10:17:04 No.11284224 Report

Quoted By:

Yeah basically it's about knowing all the possible ways to do it. You need different stuff for different calculations.

Anonymous

Anonymous Tue 07 Jan 2020 11:53:01 No.11284379 Report

Quoted By: >>11286308

>How do you solve something like 12*14 in your head?

13^2-1^2 and I remember that 13^2 is 169

Anonymous

Anonymous Tue 07 Jan 2020 11:58:49 No.11284384 Report

Quoted By:

Most people factor the numbers as some anons already said, if you want to get good at it just practice a lot, e.g. whenever you are bored think of two numbers and try to multiply them in your head.

Anonymous

Anonymous Tue 07 Jan 2020 12:13:13 No.11284397 Report

Quoted By:

>>11283659
It’s all about finding the best shortcut.
First method I went to for this problem is 6*7*4, but it’s all about finding what works for you.

Anonymous

Anonymous Wed 08 Jan 2020 03:32:01 No.11285994 Report

Quoted By:

12^2=144
12*2=24
144+24=168

Anonymous

Anonymous Wed 08 Jan 2020 03:33:58 No.11285998 Report

Quoted By: >>11285999

>>11283659
I went 120+48(120+40)(168).

Anonymous

Anonymous Wed 08 Jan 2020 03:35:05 No.11285999 Report

Quoted By: >>11286002 >>11286303 >>11286310

>>11285998
Just be like Warren Buffet and do quick maths every minute. What is 60% of 1400?

Anonymous

Anonymous Wed 08 Jan 2020 03:36:29 No.11286002 Report

Quoted By:

>>11285999
6*10+24=84
84000/100
840i go like this in 30 secs max.

Anonymous

Anonymous Wed 08 Jan 2020 05:31:07 No.11286161 Report

Quoted By:

I would have done 12*12 + 12*2

I just break it up in my head, you have to have learned times tables pretty young

!AnOnYMous2

!AnOnYMous2 Wed 08 Jan 2020 05:46:56 No.11286191 Report

Quoted By:

>>11283659
>>10*14 + 2*14 =168
Ya that or 12*12 which is already know is 144 + 24
Fuck doing that in your head. Just always have access to a calculator. It's not like you'd forget how to do this in your head. You're just psyching yourself out.

Anonymous

Anonymous Wed 08 Jan 2020 07:01:36 No.11286286 Report

Quoted By: >>11286298 >>11286308

>>11283659
13 * 13 - 1
BOW

Anonymous

Anonymous Wed 08 Jan 2020 07:11:04 No.11286298 Report

Quoted By:

>>11286286
same

Anonymous

Anonymous Wed 08 Jan 2020 07:14:19 No.11286303 Report

Quoted By:

>>11285999
Seems like a long way to go about it, but
50% of 1600= 700
700/5= 140 (another 10%)
700+140 = 840

I think I have the same problem as OP. Why did my mind instinctively go through that slower process when
50%=700
10%=140
700+140=840

Anonymous

Anonymous Wed 08 Jan 2020 07:17:14 No.11286307 Report

Quoted By:

Any good books for this? I know about 'be a math magician!' but it's kind of cheese/overly wordy. I also know about MIT's art of estimation book.

Anonymous

Anonymous Wed 08 Jan 2020 07:19:33 No.11286308 Report

Quoted By: >>11286314 >>11287216

>>11284379
>>11286286
Does this work for all numbers around squares?
4*6 = 5^2 -1 = 24
19*21 = 20^2 -1 = 399
Huh. Holds up so far anyway..

Anonymous

Anonymous Wed 08 Jan 2020 07:23:08 No.11286310 Report

Quoted By:

>>11285999
60% * 1400 == 1400 * 0.6 == 140 * 6 == 140 * (3 * 2)
140 * 3 = 420,
420 * 2 = 840

Anonymous

Anonymous Wed 08 Jan 2020 07:24:48 No.11286314 Report

Quoted By:

>>11286308
yes
$x^2 = (x-1) * (x+1) +1$

Anonymous

Anonymous Wed 08 Jan 2020 07:29:38 No.11286320 Report

Quoted By:

>>11283659
12*12=144
+
12*2=24
24+144=168

Anonymous

Anonymous Wed 08 Jan 2020 07:31:17 No.11286325 Report

Quoted By:

$==$

Anonymous

Anonymous Wed 08 Jan 2020 19:38:40 No.11287216 Report

Quoted By: >>11287254

>>11286308
It does. Even something like 93*107 becomes an easy 10000-49.

Anonymous

Anonymous Wed 08 Jan 2020 19:49:04 No.11287232 Report

Quoted By:

fun thing about squares, if you wanna find the square of 13 and you only know the square of 12, just get the square of 12,144 and add 1 + 2*12, this, ofcourse, is because the second difference of the nth term sequence is n^2 is 2 and the difference between 0^2 and 1^2 is 1, so you add 1 (first difference) + 2(second difference) * the amount of the second difference has been applied

Anonymous

Anonymous Wed 08 Jan 2020 19:55:37 No.11287245 Report

Quoted By:

>>11283659

12*15-12

Anonymous

Anonymous Wed 08 Jan 2020 20:02:17 No.11287254 Report

Quoted By: >>11287439

>>11287216
So the general rule would be?

$x^2 = (x+y)(x-y) + y^2$

Anonymous

Anonymous Wed 08 Jan 2020 21:42:58 No.11287439 Report

Quoted By:

>>11287254
Yes!

Anonymous

Anonymous Wed 08 Jan 2020 21:59:56 No.11287495 Report

Quoted By:

>>11283659
Hey OP, here's a git gud for you.
In case the math went too quickly, check the digital sum and the last one or two digits.

For example: 214+783+965=1857 Is it correct?

2+1+4=7----------------->7
7+8+3=18-->1+8=9-->0
9+6+5=20-->2+0=2-->2
The sum total of 7, 0 and 2 makes 9, which you can count it as zero. The digital sum of 1857 is 3 which means the whole sum is wrong.

If it were correct, the last two digits must be _14+_83+_65=_62. So the actual sum has to be 1962 to match the digital sum of zero it is expected to be (it's not 1862 because the digital sum is only 8).

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