Multiplication Trick — Multiplying by 5

Time for a math trick …

Q:  How do you multiply an even number by 5 in lightning speed?

A:  Divide the number by 2, then tack on a “0.”

Example:   5 x 24

Divide 24 by 2 to get 12.

Tack a “0” onto 12 to get 120. Presto, nothing up your sleeve. It’s that easy.

Why does it work? Hint: Think about how we multiply by 10. Then think about how multiplying by 5 compares to multiplying by 10.

Rotated version of File:Symbol support2 vote.svg.

Image via Wikipedia


Try these for fun (answers at bottom of post):

a)  5  x  16

b)  5  x  8

c)  5  x  28

d)  5  x  64

e)  5  x  142

f)  5  x  2,468

g)  5  x  6,042

h)  5  x  86,432

j)  5  x  888,888


Answers:

a)  5  x  16 = 80

b)  5  x  8 = 40

c)  5  x  28 = 140

d)  5  x  64 = 320

e)  5  x  142 = 710

f)  5  x  2,468 = 12,340

g)  5  x  6,042 = 30,210

h)  5  x  86,432 = 432,160

j)  5 x 888,888 = 4,444,440

k)  5  x  2,486,248 = 12,431,240

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One Response to “Multiplication Trick — Multiplying by 5”

  1. Dorothy says:

    I’ve known this trick forever too. And I bunch of otrhes too. One of my favorites is for calculating an 18% tip on a bill. 18% is just 20%-2% (and 20% is twice 10%).So on a $34.20 bill you take 10% ($3.42), double it ($6.84) and minus 10% of that amount ($6.84-$0.68 = $6.12). With practice, you can manage this fairly quickly. No more reaching for your cell phone’s calculator unless you’re feeling really lazy.There’s a great book out there (Arithmetrics) that I love. As a teacher, I use some of those tricks with my students all the time. I am not sure it covers the trick you mention above (it probably does but most of the ones I use with my students are for addition and subtraction).

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