In resonance tube experiment, 3^rd resonance occurs at 120 cm and 5^th resonance occurs at 220 cm. Find the end correction (in cm).

$\frac{5\mathrm{\lambda }}{4}={\mathrm{l}}_1+\mathrm{e} ...\left(\mathrm{i}\right)$
$\frac{9\mathrm{\lambda }}{4}={\mathrm{l}}_2+\mathrm{e} ....\left(\mathrm{ii}\right)$
From (i) and (ii)
$\mathrm{\lambda }={\mathrm{l}}_2-{\mathrm{l}}_1=220-120=100\mathrm{cm}$
From (i), end correction 
$\mathrm{e}=\frac{5\mathrm{\lambda }}{4}-{\mathrm{l}}_1=\frac{5}{4}\times 100-120\Rightarrow \mathrm{e}=5\mathrm{cm}$