The most common compound measure that we need to know about is average speed, which is a measure of the distance travelled during a certain time.

The easiest way to do calculations involving distance speed and time is to use the “distance speed time triangle” to remember the formula we need to calculate each one of these:

Let’s try doing some calculations using this triangle:

Other compound measures

Speed is not the only compound measure – you will notice several triangles in your book for different compound measures. All of them work in the same way as the “distance speed time” triangle. You just have to check which part of the triangle you need to calculate and then read off the formula from the other parts of the triangle:

And don’t forget that you must be careful with units. If a question asks you to calculate speed in “miles per hour”, it is no good dividing the number of miles travelled by the number of minutes taken. What must you do instead?

Exercise

Let’s practice working with compound measures in exercise 11A on page 164 of the textbook:

Below are the answers:

Value for money

We use a fairly similar technique when we want to work out the value for money of different products which have different sized packages and different cost. Basically, we divided the cost by the size of the package to calculate the price per unit that we are paying. This is effectively a compound measure like the ones that we have looked at above. The items with the lowest price per unit are the best value for money, and the one we would often want to buy. (In what situations would we not want to buy the best value for money product.

Exercise

Let’s practice calculating the best value for money product in the following exercise (11B in the textbook):

Below are the answers: