How do you do multiplication in your head?

[notfred]

I think (this is conjecture, I don't know, it's why I asked the question) that a lot of people try to do math in thier heads the same way they do it on paper. basically, if they have 19x12, they do this:

2x9 = 18
carry the 1
2x1 = 2
add the one
top row is 38

add a zero to the bottom row
1x9 = 9
1x1 = 1
bottom row is 190

Add:

0+8 = 8
9+3 = 12
carry the one
1+1 = 2

final answer = 228

Now, I'm guessing that using this method is what makes a lot of people really slow at math in thier heads.

Now, personally, if I'm doing math in my head, I do it differently, I do 12x19 like this:

19 is close to 20. 20 is 10 x 2

12x10 = 120 (add a zero, easy to do in your head)
120x2 = 240 (doubling is also pretty easy in your head, I think)

Then, subtract 12, cause we started with 19 and not 20

240 - 12 = 228.

It's definitely a lot faster than the earlier method, I'm jsut curious to see if anyone else uses some other method of speeding up math in thier head, or whether most people do what I do, or use the same method as on paper. Some people jsut seem really slow at doing math (the proverbial cashier who can't count change), and I can only figure it's because they're trying to do it the same way as on paper.

 

[WinkOsmosis]

19x12? I would do 20x12, which is 240, then subtract 12. 228.

 

[Yossarian]

I dont' like subtraction so I usually do 19x10 and add 19x2.

 

[AccruedExpenditure]

Originally posted by: WinkOsmosis
19x12? I would do 20x12, which is 240, then subtract 12. 228.

Percisely

 

[Orsorum]

Originally posted by: WinkOsmosis
19x12? I would do 20x12, which is 240, then subtract 12. 228.

I do this method as well.

 

[TommyVercetti]

Yeah I use that second method. But whenever I have to multiply in my head, it's always when i am out shopping. So I calculate % off discounts and tax like that. I make it a bit simpler though. If the price is $11 and has a 6% discount, I just use round up to the closest workable numbers. I would round $11 to $10 and 6% to 5%. That makes things easier and just gives me an estimate.

 

[DrPizza]

I do it a variety of ways, depending on the problem..... but, I agree with wink on that one.

Something like 15*17, I would do 16 squared minus 1. = 256-1 = 255.

(it's from the form (x-1)(x+1)= x^2 -1 )

 

[hevnsnt]

It is official. This is the GEEKI-EST Thread EVER

 

[isaacmacdonald]

20x12
- 12

or a more long winded one:

19x10
+ 19x2

 

[Mr N8]

The second way is how I generally do it. However, the majority of my multiplication is (milage) x .36, so then I usually use a calculator.

 

[Yossarian]

Originally posted by: DrPizza

Something like 15*17, I would do 16 squared minus 1. = 256-1 = 255.

(it's from the form (x-1)(x+1)= x^2 -1 )

You're a maniac for even considering that.

 

[DaveSimmons]

Originally posted by: WinkOsmosis
19x12? I would do 20x12, which is 240, then subtract 12. 228.

exactly -- same general idea as notfred though, to calculate something "easy" and "close" to the exact question, then add or subtract the difference:

188x12 = 1880 + 360 + 16 = 2256.

 

[Yax]

19x12

Yes, its like this:

19 is 10 + 9.
12 is 10 + 2.

10 times 10 is 100.
9 times 2 is 18.
so,
add the two to get 118, then add 100 and finally add 10 to get 228.

See, no subtraction involved.

 

[dighn]

i add/subtract from left to right if it's simple, eg like this:
19x12

19 x 2, 1*2 = 2 but 2*9 carries so it's 3 in tenth place, 9*2 = 18, so 8 in oneth place
then 190 + 38, 3+9 carries so 2 in hundred place, 8+0 no carry so 2 in tenth place, then 8 in oneth place. checking for carry just takes a glance and even though there is some redundency but it takes almost no time if you get used to it. anyway for me it's sometimes faster than getting the answer from right to left then reverse it into natural speaking/writing order. i dont use this for more digits though cuz then it just gets too slow

 

[Saltin]

It just gets done.

 

[isaacmacdonald]

Originally posted by: cheapbidder01
19x12

Yes, its like this:

19 is 10 + 9.
12 is 10 + 2.

10 times 10 is 100.
9 times 2 is 18.
so,
add the two to get 118, then add 100 and finally add 10 to get 228.

See, no subtraction involved.

you're afraid of subtraction aren't you?

 

[FrogDog]

I do it the same way you do, notfred. I just look for the easiest calculation by adding/subtracting one or two to one of the numbers, and then subrtract/add one or two x the other number.

 

[Howard]

19x10 + 20x2 - 2

 

[CPA]

Originally posted by: isaacmacdonald
20x12
- 12

or a more long winded one:

19x10
+ 19x2

I would do either of these, but would probably have done the second one first, because 19 is the first number in the operation.


<---CPAs have 31337 multiplication skills

 

[Pilsnerpete]

Originally posted by: dighn
i sometimes go from left to right eg like this:
19x12

19 x 2, 1*2 = 2 but 2*9 carries so it's 3 in tenth place, 9*2 = 18, so 8 in oneth place
then 190 + 38, 3+9 carries so 2 in hundred place, 8+0 no carry so 2 in tenth place, then 8 in oneth place. the advantage of this method is i get the results from left ot right in natural speaking/writing order. gets too messy for more digits thoguh

wow, whatever works

 

[dighn]

Originally posted by: Pilsnerpete

Originally posted by: dighn
i sometimes go from left to right eg like this:
19x12

19 x 2, 1*2 = 2 but 2*9 carries so it's 3 in tenth place, 9*2 = 18, so 8 in oneth place
then 190 + 38, 3+9 carries so 2 in hundred place, 8+0 no carry so 2 in tenth place, then 8 in oneth place. the advantage of this method is i get the results from left ot right in natural speaking/writing order. gets too messy for more digits thoguh

wow, whatever works

it sounds like a lot of work but it's really simple if you get used to it but then it depneds on the situation. this particular multiplication happnes to be very easy also if it was a test i would use one of the other shortcuts for simplicity but my obesessiveness forces me to do more work otherwise...

 

[SOSTrooper]

I do it the Hong Kong way...

Lets say it's 26x34

....26 .......I drop the 0 down below the 6 and 4
..X34 .......Then I multiply 3 to 26 to get 780
-------
..780 .......Now I multiply 4 to 26 to get 104
+104 .......Finally I add 780 to 104
-------
..884

 

[Shawn]

I cannot do multiplication in my head.....

 

[Encryptic]

Originally posted by: iloveme2
I cannot do multiplication in my head.....

Tell me about it. I'm bad at math in the first place, so I usually have to use a calculator or work it out on paper if it's much more than a couple of single digit numbers.

 

[DrPizza]

Originally posted by: PipBoy

Originally posted by: DrPizza

Something like 15*17, I would do 16 squared minus 1. = 256-1 = 255.

(it's from the form (x-1)(x+1)= x^2 -1 )

You're a maniac for even considering that.

Nah, not a maniac, just way better than most people at mental arithmetic. It only takes practice though. The more you rely on a calculator, the weaker your ability to do mental math becomes. It's fun to beat people who are using a calculator... especially sales clerks. Or better, to get out the exact change long before they're done adding up the prices and calculating the sales tax.

Another way to do things mentally, say for something like 26 * 47 would be to also reduce it to an algebra problem
(20 + 6)(40 + 7) use FOIL
I would: 20*40 = 800
40*6 = 240, running total is 800...900...1000...1040
20*7 = 140, running total 1040...1140...1180
6*7=42, running total 1180...1220...1222 Done.

Yet another way to do 26*47 would be
26*47..... 13*94
940 + 3*94
940 + 300 -18
1222

Either way, takes about 2-3 seconds mentally. (for me)