Decimal Multiplication

Today we looked at decimal multiplication. Again, it is very similar to standard multiplication. If you need a refresher on this (as it should have been covered in primary school) this websites have some great information and examples.

Multiplication: http://www.mathsisfun.com/numbers/multiplication-long.html

This next website is a link to the video. Once you have watched the video you can click on the green "Practice this concept" button in the top right corner to get a few practice problems. These practice problems are great because if you are getting stuck, you can click on a "Hint" button, and it will help you out a little. The website is: http://www.khanacademy.org/math/arithmetic/multiplication-division/multi_digit_multiplication/v/multiplication-6-multiple-digit-numbers


The way I multiply decimals is slightly different compared to others I have seen, however I have found it to be quite an easy method. The first step I take is to look at the problem and count the number of numbers after the decimal points in the numbers I am multiplying. I then record this off to the side because I will use it later. Next I completely ignore all the decimals and just multiply like I normally would if they were whole numbers. When I get an answer, I then need to modify this slightly. Off to the side I have recorded the number of digits after the decimal points in my question. I recorded this, because this is the number of digits that need to be after my decimal point in my answer. I then put the decimal point back into my answer in the appropriate spot so that it has the same amount of number after the decimal point as my question did. This is then my final answer.
I understand this sounds very complicated, however hopefully I can work through a few examples and it will make a little more sense.


Clicking on any of these images will enlarge them if you are having trouble viewing these.

Example 1:

Example 2:

Example 3:

If you are still having troubles, the following website is also handy

Multiplying decimals: http://www.mathsisfun.com/multiplying-decimals.html

To practice this, look in your booklet is section 4.5 "Decimal Multiplication". I want you to do the following problems:

1 and 3 -> first columns only 

7, 8, 11

Extension: 17 and 18

Our next class for maths is on Tuesday the 28th of July. I need you all to complete the first columns of 1 and 3, and then either 7, 8, or 11 by then for homework. 

As always, if you have questions or comments feel free to comment on this post, email me, or see me in person.