At constant temperature on increasing the pressure of a gas by $10\%,$ its volume will decrease by

${\text{P}}_1=\text{P}, {\text{then P}}_2=\text{P}+\text{1}0\%\text{ of P}=\text{1}.\text{1P}$

According to Boyle's law PV = constant 

${\text{P}}_1{\text{V}}_1={\text{P}}_2{\text{V}}_2{\text{,V}}_{\text{2}}\text{=1}0/\text{11}\left({\text{V}}_1\right),$

$({\text{V}}_2-{\text{V}}_1)/{\text{V}}_1=-\text{1}/\text{11×100}=–\text{9}.0\text{9}\%$

Its volume decrease by $\text{9}.0\text{9}\%$