At constant temperature on increasing the pressure of a gas by 5% will decrease its volume by:

If $P_1=P$ then $P_2=P+5\% of P=1.05P$

From Boyle’s law PV = constant

$\therefore \frac{V_2}{V_1}=\frac{P_1}{P_2}=\frac{P}{1.05P}=\frac{100}{105}$

Fractional change in volume $=\frac{\Delta V}{V}=\frac{V_2-V_1}{V_1}$

$=\frac{100-105}{105}=-\frac{5}{105}$

$\therefore$Percentage change in volume $\frac{\Delta V}{V}\times 100\%=-\frac{5}{105}\times 100\%=-4.76\%$

i.e, volume decrease by 4.76%