According to Boyle's Law (derived from the Ideal Gas Equation $pV = nRT$ at constant temperature ), pressure is inversely proportional to volume ($P \propto 1/V$ or $P_1V_1 = P_2V_2$). Let initial pressure be $P$ and initial volume be $V$. New pressure $P' = P + 0.05P = 1.05P$. Using the relation $PV = P'V'$, we get: $V' = \frac{PV}{P'} = \frac{PV}{1.05P} = \frac{V}{1.05}$. The decrease in volume is $\Delta V = V - V' = V - \frac{V}{1.05} = V(1 - \frac{1}{1.05}) = V(\frac{0.05}{1.05})$. The percentage decrease is $\frac{\Delta V}{V} \times 100 = \frac{0.05}{1.05} \times 100 \approx 4.76\%$.