4.2 Calculations using decimals
When you make any calculation with decimals – that is, addition, subtraction, multiplication and division – you may use a calculator. When using a calculator it is very important to make sure that the decimal point is in the correct place. If you don’t, you’ll get the wrong answer.
Now try the following activity. Remember to check your answers once you have completed the questions.
Activity 13: Using decimals
 You buy a box of corn flakes for £2.65 and a bottle of milk for £1.98.
 a.What is the total cost of these items?
 b.You pay for them with a £5 note. How much change should you get?
 A patient’s height and weight are used to calculate their Body Mass Index (BMI). These measures are often decimal numbers.
 a.What is the patient’s weight in the image above?
 b.What is the patient’s height (in metres) in the image above? Multiply the height by itself.
 c.Divide your answer to part a) by your answer to part b). This will tell you the BMI of the patient.
 Convert 6.25 m to cm.
Answer
 The answers are as follows:
a.Add the cost of the two items:
(Keep the decimal points in line.)
The total cost of the items is £4.63.
b.Take away the total cost from £5:
You should get 37p change from £5.

 a.The patient's weight is 40.5 kg
 b.The patient’s height is 1.5 m. Therefore 1.5 × 1.5 = 2.25
 c.40.5 ÷ 2.25 = 18.
So the BMI of the patient is 18.

To convert 6.25 m into cm, you need to multiply the amount by 100.
 6.25 × 100 = 625
So the answer is 625 cm.
Summary
In this section you have learned about how:
 the value of a digit depends on its position in a decimal number
 to approximate answers to calculations involving decimal numbers
 to add, subtract, multiply and divide using decimal numbers.
This will help when working with money and measurements.