# multiplying 2 numbers

• Feb 4th 2013, 09:21 AM
terminator
multiplying 2 numbers
523 * 489 using mental math

523 * 500 = 52300 + 52300 + 52300 + 52300 =

11 less groups... sounds complicated. Any other ideas?

Thanks,

Terminator
• Feb 4th 2013, 10:17 AM
HallsofIvy
Re: multiplying 2 numbers
500 is half of one thousand so 500 times 523 is 1/2 of 523000. Can you do 523000/2 in you head?

489 is 11 less than 500 so after you have 500 times 523, you still need to subtract 11 times 523. What is 10 times 523? Can you subtract that from the 523000/2 you already have? The subtract it once more.
• Feb 4th 2013, 10:55 AM
terminator
Re: multiplying 2 numbers
anything easier? I cannot do 523000 in my head...
• Feb 4th 2013, 12:14 PM
Soroban
Re: multiplying 2 numbers
Hello, terminator!

Quote:

$\displaystyle 523 \times 489\;\text{ using mental math}$

I might be able to do this mentally ... under threat of death.

$\displaystyle (523)(489) \;=\;(500+23)(500-11)$

. . . . . . . . . . .$\displaystyle =\;500^2 - (11)(500) + (23)(500) - (23)(11)$

. . . . . . . . . . .$\displaystyle =\;500^2 + (12)(500) - (23)(11)$

. . . . . . . . . . .$\displaystyle =\;250,\!000 + 6,\!000 -253$

. . . . . . . . . . .$\displaystyle =\;256,\!000 - 253$

. . . . . . . . . . .$\displaystyle =\;255,\!747$

• Mar 15th 2013, 12:33 PM
SVCrankson
Re: multiplying 2 numbers
firstly;
you can do this

532
x 489
----
4788
4256
2128
----------
260148
-----------
9x2=18
9x3=27
9x5=45

8x2=16
8x3=24
8x5=40

4x2=8
4x3=12
4x5=20

i believe this method is very easy and you just have to pick one number from the bottom and multiply it by all the numbers above
the do the same for the next bottoms number and after all that, you add all of the answer

let me know if you do not understand!!

(Smile)(Smile)(Smile)(Smile)(Smile)(Smile)(Smile)( Smile)
• Mar 15th 2013, 02:07 PM
HallsofIvy
Re: multiplying 2 numbers
Quote:

Originally Posted by terminator
anything easier? I cannot do 523000 in my head...

Seriously? 5 divided by 2 is 2 with remainder 1. 12 divided by 2 is 6 with remander 0. 3 divided by 2 is 1 with remainder 1 and 1000 divided by 2 is 500:
523000 divided by 2 is 261500.