Fast Math Trick - How to multiply 2 two digit numbers - why it works!

[epixsavior]

Dude, you rock. I've learned more from your videos in one day than I did in 4 years of American High School. You're helping on my road to success.

[danielbair]

Out of curiosity, why don't you round the bigger number, 28, to the nearest ten? And multiply 15 * 30. That makes it a much easier problem, as you could do 15 * 3 and then add the zero to get 450. From there, just subtract 2 * 15 which is 30, and you would get 420? Just curious

[snowfolk]

Your lessons are fantastic and the humour is great. Thanks!

[jobsucks]

To do this mentally I would multiply the 10s first and do the 1s last. Keep a running total in my head as I go.

For the example (15x28); 10x20=“200”, 20x5=100 +200=“300”, 10x8=80 +300=“380”, 5x8=40 +380=“420”
The numbers in “quotation” are my running total.
No carrying involve

To teach this to kids for math class where they require to show their work is easy too. Instead of using the standard algorithm we learned in school we can also use the above method. Just need to show each step. Only difference is instead to keeping a running total you write out each product then add everything at the end.
200+100+80+40 = 420

200 (10x20)
100 (20x5)
80 (10x8)
40 (5x8)
——–
420

[benadams]

I think part of the reason people have difficulty with multiplication is that they don't understand it's groups of ___ (he said the phrase briefly).

I, and someone else said it too in a comment, round one number to a "10" to make it easier (for me).

15 X 30= 450

28 is 2 less than 30, so

15 X 2 = 30

So, 450 - 30 = 420.

It's too bad that teachers in schools don't have time to teach something to someone in a way that their brain just "gets it."

I understand the way I just demonstrated it, while others are going "huh?"

[ThomGustavsson-irlt]

Here's how i would do 15 x 28: First ill round one of them to a 10, then 15 x 30 = 450. 450 - 15 x 2 = 420. 

[Chrisangel]

I love your videos man, I got all a's on my testes

[peterbarnie]

Please how can use the number line to calculate 74×13 thank you

[choochoochooseyou]

For some reason I get 15x28 as 320 instead of 420 and can't see what I'm doing wrong. Any help guys?

[davidcarnell]

fairly clumsy, the fastest method is distribution: 10 x 28 + half of that or 5 x 28 = 280+140 = 420

[jker]

Any tips on multiplying 3 digits by 2?
425
X42 ?

[skindiver]

I cant get this to work with so many numbers I have given up try 78 x 29 ???

[smdb]

I did understand what you have teach but when I multiply the number made my me with the same trick you have teach . But it doesn't

[martheworld]

You are really smart but go a little bit slow when you doing a video please

[aalieshakenon]

how did you get 10 im confused and how did you get 20 also

[adrianearl]

the answer to the first question is 420... coincidence?!?!? lol

[muskanhada]

It's y humble request please try to speak

[lachlanmorgan]

for some reason it isnt working for 64 x 64 can someone help me out