How to Do Math In Your Head : More Fast Multiplication Techniques

Learn more fast multiplication techniques to do in your head in this free video series. Get mental math tips from these online tutoring lessons.

Expert: Sean Salazar
Bio: Sean Salazar has taught in both Schools and Community Centers, specializing in Math and English.
Filmmaker: sean salazar

Category:

Science & Technology

Tags:

• education
• head
• quick
• math
• mathematics
• problems

[LivOlivia1]

that method doesnt work well unless the numbers youre multiplying are actually in the 90s

[TomKaren1994]

94 x 91

90x90 + 90(4+1) + 4x1 =

8100 + 450  + 4 =

8554.

Just multiply left to right. That's the way I've always done it.

His technique is based on the trick to find the hundreds place quickly.

(100 - A)(100 - B) =

(100)(100) - 100(A+B) + AB

(100)(100 - (A+B)) + AB

To find (100 - (A+B)), just take one number, already either A or B less than 100, and subtract the difference of the other number and 100. Put that value in the hundreds place. It's no trick. It's math.

[xJustGoAwayx]

is the beggining recorded with a potatoe????

[MinecraftPuzzleMaps]

Stupid method, since it's pretty much useless (or at the least very hard) for lower numbers.

[79ward]

Sacred geometry can explain and solve all of this.

[SwissMexProductions]

Thanks man very usefull. Seems like we forget these things very easily

[machineryProducts]

thanks dude i have a test tomorro and you really helped me thanks

[KnowledgeIsPowerKiP]

@OhWowLovelyXx0 Even if the number is really big you can just break it up

lets do 387x513.

=387x500 +387x13=500x300 +500x87+387x13=150000+43500+30­0x13+87x13

=150000+43500+3900+87x10+87x3=­197400+870+261=197400+1131=198­531

and no i did not use a calculator

[KnowledgeIsPowerKiP]

When I do these I just break them up into easier multiplications,

For example

91x94 = 91x90 + 91x4=8190+364=8554

[xXShonomercyXx]

Calculators are wonderful!

[OhWowLovelyXx0]

Ohkay. But what if the number is passed 100 like what if it's 355 do you round that to 400 or 300?!