How to Multiply

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Posted in HomeEducation

Introduction: How to Multiply

About: The answer is lasers, now, what was the question? If you need help, feel free to contact me. Find me on Reddit, Tumblr and Twitter as @KitemanX

There are several useful methods for multiplying.

This one is one of the most space-consuming, but is also one of the easiest, as it only requires you to know your tables up to 9x9.  This makes it especially useful for KS2 or less-able KS3 students (age 9+)

The rest is adding.

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Step 1: The Grid Method.

The method has several names, but is most often called the grid method.

To multiply two numbers together, the numbers are first broken down into their component place-value chunks.

For instance, let us multiply 47 by 68.

"47" is actually "40 + 7" and "68" is "60 + 8".

These numbers are written into a grid, as in the illustration below:

Step 2: Multiply the Rows and the Columns.

It's almost easier to show than describe.

Ignoring the zeros, multiply the digits at the top of the columns with those at the left of the rows.

4x6 = 24

4x8 = 32

7x6 = 42

7x8 = 56

Now we add the zeros back on - 24 gains a zero from the 40, and one from the 60, so becomes 2400.

Similarly, 32 becomes 320 and 42 becomes 420.

Step 3: Adding.

That's the hard part done.

All you have to do now is add up the four numbers in the grid. Remember to be careful about place value, and align them up to the right.


Step 4: You Want More??

You've had the basics - this method can be extended to multiplying any two number of any length.

It is possible to use it to multiply more than two numbers, but you need to work them out as you go along (for example, 23x46x17 would need you to work out 23x46 and then multiply that result by 17).

You are not just limited to two-digit numbers - here are a pair of three-digit numbers worked out on a scrap of paper.

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    21 Comments

    No - that linked method looks like a variation of Napier's Bones.

    It's supposed to be for kids who can't do the traditional "columns" method, but it is popular with all our kids, and I have even caught our head of maths using it.

    I used this method for my maths GCSE yesterday - but I had been told it wasn't the 'right' way - so I did the other method next to it

    As long as both methods gave the same result...

    heh, when I was a kid, I never paid attention in class, and came up with my own way of multiplying that was very similar

    96
    x 47


    42
    630
    240
    3600

    4512

    I got points taken off for not doing it right :-(

    2 replies

    The current policy in the UKin my school in my lessons is "if it works, it works".

    I don't mind how you get to the right answer, as long as you know how you got there, and could get there again.

    I normally find the right answer, then it runs and hides from me.

    You forgot If all fails, Just use a calculator.

    OoooOh! I thought this was an ible about unprotected......... Nevermind.

    4 replies

    Leave it to the clergyman to make this about procreation!

    (Maybe I could leave it to the clergyman to lead the voting?)

    Wow, I've never seen it done that way. But then...I was a liberal arts major. ;)

    Nice one! Ive never seen this type of multiplying... Its very cool. You got my vote!

    1 reply