**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.

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

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).