These two are easy to work out if you remember the triangle trick:

**Density**

Density is mass per unit volume. It is written, for example, like this g/cm3.

Density = mass/volume

Volume = mass/density

Mass = density x volume

Any easy way to remember this is with this triangle:**Speed**

Speed is distance travelled over a unit of time. So km per hour for example.

Speed = distance/time

Time = distance/speed

Distance = speed x time

Again you can use this triangle to remember this: