Glossary
The engineering industry requires precision and accuracy. Decimal points are used in measurements and calculations when we need to be precise with length, volume, area or time.
Using a decimal point in a number is one way to show a number that is less than 1. An example of a decimal number less than 1 is 0.125.
Notice the way the number starts with a 0? This is to make sure that you recognise that the decimal point is there. The decimal point shows that the number is more than 0 but less than 1.
When the numbers you add or subtract have decimals it is important to make sure the decimal points line up when setting the problem out:
Eg: 
When you set the problem out it is a good idea to fill any spaces with
a 0 so that you don't get confused by empty columns when adding or subtracting
Eg: 
Multiplying numbers with decimals is much the same process as multiplying numbers without decimals.
To start with, ignore the decimal points and carry out a normal multiplication.
Once you have calculated an answer, add up the number of digits after the decimal points in the original two numbers. For example, in the calculation of 172.13 X 15.375, there are 5 digits after the decimal points altogether. This means the final multiplication answer also needs to have 5 decimal places.
Step 1: Normal multiplication (ignore decimals)
Step 2: Put decimal points back in the original numbers to work out how many have to be included in the answer
When dividing numbers which have decimals care must be taken with setting out.
If you are dividing by a whole number (eg 30.680 ÷ 8) then use your normal process, but be careful to place a decimal in your answer immediately above the decimal in the number being divided.
Eg: 
When dividing a decimal by a decimal (eg 2.17 ÷ 0.31) you must first change the decimal number you are dividing by to a whole number. You can do this without affecting your calculation if you multiply both numbers by the same multiple of 10. For instance, if you are dividing by a number with 2 decimal places, you need to first multiply both numbers by 100.
Eg: 2.179 ÷ .31
0.31 x 100 = 31 A whole number
2.179 x 100 = 217.9
Now the decimals have gone from the number you are dividing by, the division
problem becomes 217.9 ÷ 31 and can be calculated in the normal way.
Complete
this decimal point activity sheet (Word 91 kb)
