Introduction: How to Multiply Decimals

Picture of How to Multiply Decimals

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

Picture of 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

Picture of 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

Picture of Digits

Now, you must add the digits.

Step 4: Magic!

Picture of 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

Picture of 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.


STEMsheets (author)2014-09-04

Very interesting, thanks for sharing this technique. This website has some multiplying decimals worksheets to practice with:

maxyor (author)2012-08-05

YOU NEED MORE STEPS and examples

HMice (author)2012-06-29


Chaucermeow (author)2011-06-13

"Now, you must add the digits."

I am not seeing any addition in this particular step.

LiLGq (author)2011-03-03

i dont understand.....

NatNoBrains (author)LiLGq2011-03-04


nfk11 (author)2011-03-03

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

MCzone (author)2011-02-28

Explain the *pundent* square for integers

NatNoBrains (author)MCzone2011-03-01


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.

NatNoBrains (author)2011-02-28

OK Thanks

cokecola (author)2011-02-27

I think it's COOL!

NatNoBrains (author)cokecola2011-02-28


About This Instructable



Bio: I am not your average teenager. I like a lot of stuff, and will be posting some more stuff soon. I spend a lot of ... More »
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