The following table gives the distribution of the life time of 400 neon lamps :

$\frac{\mathbf{N}}{\mathbf{2}}=\frac{\mathbf{3 9 9}}{\mathbf{2}}=199.5$

Median class $=3000-3500$

Median $=\ell+\left\{\frac{\frac{\mathbf{N}}{\mathbf{2}}-\mathbf{C}}{\mathbf{f}}\right\} \times \mathbf{h}$

$=3000+\left\{\frac{\mathbf{1 9 9 . 5}-\mathbf{1 3 0}}{\mathbf{8 5}}\right\} \times 500=3408.82$

Hence, median life time of a lamp 3408.82 hrs.