While speed, distance and time are the most common **compound units** you will encounter in exams, you still need to be able to find others too.

In this activity, we will be focusing on here is **density**, **mass** and **volume**.

**Density **is a measurement of how heavy something is for a given unit of** mass**; (effectively, the *denser *an object is, the *heavier* it is.)

The most challenging element of calculating with density, mass and volume is in remembering the formulae you will need to apply.

**How to Remember the Formulae**

The easiest approach is to memorise the formulae triangle shown below, which we can then use to find the single formula we need to apply:

**How We Use this Formulae Triangle**

It's quite simple, just highlight the element you are trying to find (we've done it below with a red circle) and the formula to find this will use the elements which are left over:

Let's look at these formulae in action now in some examples.

**e.g. Find the volume of an object that has a density of 20 g/cm ^{3} and a mass of 500 g.**

The first step in these calculations is always to decide which element we are trying to find.

For this question, we are directly asked for the **volume**, so we need to use:

V = M ÷ D

Now we can simply plug our numbers into this formula:

V = 500 ÷ 20

The last question that we need to ask ourselves is what units of measurement we need to use.

The trick here is that they are always the same as **one of the units** provided in the question.

The density here is given in g/cm^{3}, so our volume must be **cm ^{3}**.

So Distance = 25 cm^{3}

**e.g. An object has a volume of 75 cm ^{3} and a mass of 15 g. Find the average density of the object.**

Once again, we need to decide which formula.

Don't worry that it asks for *average density *here, we can still use the same formula:

D = M ÷ V

Plugging in our numbers gives us:

D = 15 ÷ 75 = 0.2

Our units are *grams per cm ^{3}* (remember, the units

**always**are in the question).

So Density = 0.2 g/cm^{3}

In this activity, we will use the Mass, Density, Volume formulae pyramid to identify the correct single formula to apply to find a missing metric using the method demonstrated above.