1. The problem is to make $n$ the subject of the formula $PV = nRT$. 2. The formula relates pressure ($P$), volume ($V$), amount of substance ($n$), gas constant ($R$), and temperature ($T$). 3. To isolate $n$, divide both sides of the equation by $RT$: $$n = \frac{PV}{RT}$$ 4. Show the cancellation step: $$n = \frac{\cancel{P}V}{\cancel{R}T}$$ (Here, $P$ and $R$ are not canceled but this illustrates the division by $RT$.) 5. The final formula for $n$ is: $$n = \frac{PV}{RT}$$ This means the amount of substance $n$ equals the pressure times volume divided by the gas constant times temperature.