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How do I multiply three digit numbers times three digit numbers
By kenetra on Tuesday, August 10, 1999 - 02:02 am:

help me

By Chris Jefferson (Caj30) on Saturday, August 21, 1999 - 09:58 am:

Hmm.. I'll have a go..



lets say we want to multiply together.. 345 and 789, for example.

Now, does this make sense? 3*(4+5) = 3*4 + 3*5

Because this lets us write: 345*789= 345 * (700 + 80 + 9)
= 345*700 + 345*80 + 345*9

and 345*700 = 345 * 7 * 100, as 700 = 7*100
Now, the best way to write this down is as follows:

1) 345
` *789
-------------
` 3105

Here, I've written 345 * 9 = 3105 (I really hope so!) in the first line.

Now, to work out 345*80 = 345*8*10
345*8 = 2760

To multiply something by 10, the easiest thing to do is just to stick a '0' in front of them. 28 * 10 = 280 for example.

So in the next row, write 27600

If you were actually doing this, the easiest thing to do is to put a 0 at the start, then work out 345*8, shifting all the numbers along one (does that make sense?)


So this last step is 345*700

So we've already got:

` ..345
` .*789
--------
` .3105
` 27600

` 00 <- those two 0s...

Now to do some normal multiplication..

` ...345
` ..*700
--------
` 241500

You do this by normal carrying on in multiplication of 345*7=2415

So finally (!!)

` ...345
` ..*789
--------
` ..3105
` .27600
` 241500
--------
` 272205

We add up the three numbers to get the total....

Phew! Yes, this is very long, and no, there isn't really a quicker way of doing it... You'll get faster as you practice, I don't think I've done that for over 4 years.. That's scary!

Anyway, any other questions, or bits I didn't write well, then write stright back!

By Aly Crowley (P1162) on Wednesday, September 8, 1999 - 01:55 am:

..345
..789
-------
....0
...00

times the nine with all top row numbers
cross off nine, next row place a zero
times 8 by all top row numbers, cross 8 off, place 2 zeroes on the next and so forth then add them all up vwalla!!!!!

By Graham Lee (P1021) on Wednesday, September 8, 1999 - 07:32 pm:

An impressive amount of brevity.

GL

By Rachel Grant (P1159) on Sunday, September 12, 1999 - 07:09 pm:

123
456x

Firstly multiply 6 x 3. Write your answer under the 6 (remember to carry the tens!):

123
456x
..8
.1

Then multiply 6 x 2. Add the one on from the previous sum (remember to carry again!):

123
456x
.38
1

Then multiply 6 x 1 (remember to add the one!):

123
456x
738

After that you start on a new line and you add a nought because you are times by ten:

123
456x
738 - answer one!
..0

Multipy 5 x 3 remember to carry!)

123
456x
738 - answer one.
.50
1

Then multiply 5 x 2 - remember to carry and add the one:

.123
.456x
.738 - answer one!
.150
1

Multiply 5 x 1 (remember to add the 1)

.123
.456x
.738 - answer one.
6150 - answer line two.

Then you add two noughts because we're multipling by 100.

.123
.456x
.738 - answer 1
6150 - answer 2
..00

Multiply 4 x 3 (carry the one)

.123
.456x
.738 - answer 1
.6150 - answer two
.200
1

Multipy 4 x 2 - add the one

.123
.456x
.738 - answer one.
6150 - answer 2.
9200

Multiply 4 x 1 and add your answer in (nearly done)

..123
..456x
..738 - answer one
.6150 - answer 2
49200 - answer three.

Finally add all three numbers together.

______
..738
.6150
49200
_____
56088

Sorry if this is what Chris Jefferson wrote but I couldn't underastand what he wrote, but that may be because I'm only 13. But I hope this helps you (Any mistakes, sorry, I tried!)

By Chris Jefferson (Caj30) on Sunday, September 12, 1999 - 10:07 pm:

Yep, they look the same to me!

Don't worry, everyone keeps telling me all the time that I don't write clearly enough. Any time you want to make what I say clearer, feel free!