Introduction: How to Multiply Decimals

Hi,
My mum's a teacher and she found thid really cool way of multiplying decimals in a teaching book. In the book it calls it 'Italian'.
Here goes...

Step 1: Table

In this example, the sum is 35.8 times 4.6
Set out a rectangle like this:

(The dots are decimal points)
(The diagonal lines are important)

Step 2: Tens

Multiply the numbers across so 3 times 4 is 12 and the tens digit is 10, so you write 1.

(If the product of the two numbers is less than 10, leave this half blank)

Step 3: Digits

Now, you must add the digits.

Step 4: Magic!

Add the up numbers diagonally. If the sum is over 10, carry one to the next sum to the left, if it is over 20, carry two to the left, etc.

Step 5: Decimal Point

Now, to find the decimal point, find the line where both decimal points join up and go along the line.

So the answer is 164.68!

Try with other numbers and enjoy "cheating" on your maths questions.
NM

Comments

author
STEMsheets (author)2014-09-04

Very interesting, thanks for sharing this technique. This website has some multiplying decimals worksheets to practice with: http://stemsheets.com/math/multiplying-decimals-worksheet-horizontal

author
maxyor (author)2012-08-05

YOU NEED MORE STEPS and examples

author
HMice (author)2012-06-29

Huh?

author
Chaucermeow (author)2011-06-13

"Now, you must add the digits."

I am not seeing any addition in this particular step.

author
LiLGq (author)2011-03-03

i dont understand.....

author
NatNoBrains (author)LiLGq2011-03-04

Cool

author
nfk11 (author)2011-03-03

wow this is hard for me because im in 5th grade

author
MCzone (author)2011-02-28

Explain the *pundent* square for integers

author
NatNoBrains (author)MCzone2011-03-01

*Huh*

author
MCzone (author)NatNoBrains2011-03-03

My teacher taught me this for mulitpying and dividing integers. example one positive and one negative make a negative, just follow the chart.

Punnett sqare.png
author
NatNoBrains (author)2011-02-28

OK Thanks

author
cokecola (author)2011-02-27

I think it's COOL!

author
NatNoBrains (author)cokecola2011-02-28

Thanks

About This Instructable

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