Two rules of variation:

- Varies directly means we multiply
- Varies inversely means we divide

p = kr^2/qs

With p = 40 when q = 5, r = 4 and s = 6, we have:

4^2k / 5 * 6 = 40

16k/30 = 40

Cross multiply:

16k = 40 * 30

16k = 1200

Using our equation calculator, we get:

k =

**75**

So our final equation of variation is:

**p = 75r^2/qs**