Mental Multiplication Math Trick - Multiply numbers in your head near 100

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This mental multiplication math trick uses the idea of a third number - a base number - to help us multiply two numbers in your head. This series of videos first looks at multiplying numbers near 10 using base 10. The second video looks at multiplying using base 20 and 30. The third video looks at how to apply this mental multiplication trick to numbers near 50. The forth video the investigates ho to easilt mentally multiply numbers near to 100. Continue your mental math multiplication abilties here!

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I’ve been watching all your video series on multiplication and they really helped a lot in improving my multiplication skills! You’re the best math teacher I never had! Thanks so much for these videos..God bless!

robinmathews

Just starting from no 1 and working my way up.... Thanks tecmath enjoying maths again..

mrstuartgibbs

For those that know algebra, this is basically changing it to multiplying two binomials and using the FOIL method.

105x107

Change that to

(100+5)x(100+7)

The cross-adding is combining the FOI in the FOIL method. It makes it easy because your First numbers are 100.

10000
+ 700
+ 500
————
11200

So now it’s just the Last number added to that, or

5x7 = 35

11235 is the answer.

GunterJPN

i like this makes it so much faster atleast if your dealing with factors of 10 ....10, 100, 1000, 10000, and so on so if its 40, 010 x 40, 020 you would just add 200 to what ever you get by timing 400, 300, 000 by 4 ....which is 1, 601, 200, 200...so much faster this way than doing it the typical way

Farreach

Doing the math, this will also work with fractions or decimals as long as you do the multiplication correctly.

Also, I like how this is recursive

Say I wanted to do 120 × 123

Base 100:
+21, +23
14300 + 21 × 23

Base 20:
+1, +3
24 × 20 = 480 + 3 = 483

So 121 × 123 is 14300 + 483 = 14783

----
Also possible with 120 as a base, but that is harder for me to do in my head

Nerketur

Dude thanks for the math cause I'm getting those equations next week and this one

deborahovalle-vazquez

Use the "two base" method, where the secondary base is a simple multiple of the main base.
For example, use: 20 (main) & 100 (main x5), to multiply 24 by 96
So, the differences become: 24 - 20 = +4 96 - 100 = -4
Now the important bit: Before cross adding, multiply (or divide) one by that base factor, 5. Either: +4 x 5 = +20 or -4 / 5 = -4/5
Using the easier +20 gives: 96 +20 = 116 x 20 (main) = 2320
Now multiply the (unmodified) differences: +4 x -4 = -16
And finish it: 2320 -16 = 2304

RicochetRita

Thanks for this series, very good teaching :D

Black.cat._

I learnt a great concept today May 22, 2017 (IST)

DJ-cmzm

What about when you have mixed base like one number base 10 and another base 100?

islandboyw

Now, supposing we'd chosen the use the -4/5 difference. It still works, but it's a more difficult choice to do mentally.
So, 24 -4/5 = 23 1/5 = 23.2
Since we diminished this difference, we have to multiple by the secondary base of 100: 23.2 x 100 = 2320
Now the rest is the same: +4 x -4 = -16. So, 2320 -16 = 2304

RicochetRita