A gas in a container has a volume of 2000 cm3. If the pressure is reduced to 1/5 of its original pressure while the temperature remains constant, what is the new volume?

The problem describes a fixed mass of gas at a constant temperature, which means Boyle's Law applies.
Initial volume, $V_1 = 2000 \text{ cm}^3$.
The new pressure, $P_2$, is $\frac{1}{5}$ of the original pressure, $P_1$. So, $P_2 = \frac{1}{5} P_1$.
We need to find the new volume, $V_2$.

Boyle's Law states that for a fixed mass of gas at constant temperature, the product of pressure and volume is constant:

Divide both sides by $P_1$:

Multiply both sides by 5:

The new volume is $\text{10000 cm}^3$.