If an ideal gas at 100 K is heated to 109 K in a rigid container, the pressure increases by X%. 
What is the value of X?

V, n constant. 

$\begin{array}{c}\frac{{\mathrm{P}}_{\mathrm{i}}}{{\mathrm{T}}_{\mathrm{i}}}=\frac{{\mathrm{P}}_{\mathrm{f}}}{{\mathrm{T}}_{\mathrm{f}}}\Rightarrow {\mathrm{P}}_{\mathrm{f}}=\frac{{\mathrm{T}}_{\mathrm{f}}}{{\mathrm{T}}_{\mathrm{i}}}{\mathrm{P}}_{\mathrm{i}}=\left(\frac{109}{100}\right){\mathrm{P}}_{\mathrm{i}}\\ \Rightarrow {\mathrm{P}}_{\text{increases }}\mathrm{\Delta P}={\mathrm{P}}_{\mathrm{f}}-{\mathrm{P}}_{\mathrm{i}}=\frac{9}{100}{\mathrm{P}}_{\mathrm{i}}\end{array}$

$\therefore$% Pressure increases

$\begin{array}{l}=\frac{{\mathrm{\Delta P}}_{\mathrm{f}}}{{\mathrm{P}}_{\mathrm{i}}}\times 100=\frac{9{\mathrm{P}}_{\mathrm{i}}}{100{\mathrm{P}}_{\mathrm{i}}}\times 100\mathrm{\%}\\ \mathrm{X}\mathrm{\%}=9\mathrm{\%}\\ \mathrm{X}=9\end{array}$