Answer
$v=1.2\times 10^6\frac{m}{s}$
Work Step by Step
We know that;
$\frac{v}{c}=\frac{\Delta \lambda}{\lambda_{\circ}}$
We plug in the known values to obtain:
$\frac{v}{c}=\frac{0.4}{100}$
$\frac{v}{c}=0.004$
$v=0.004c$
$v=1.2\times 10^6\frac{m}{s}$
