Using Doppler effect, apparent frequency is given by $f=\left(\frac{v\pm v_0}{v\mp v_s}\right)f_0$, where $f_0=\text{actual frequency}, v_0=\text{velocity of observer and} v_{\mathrm{s}}=\text{velocity of source}$

For train $B$ leaving the station, 

$f_1=300\left(\frac{330-0}{330-\left(-30\right)}\right)=275 \mathrm{Hz}$     $\left(\because v_0=0\right)$

For train $A$ reaching the station, 

$f_2=300\left(\frac{330-0}{330-\left(30\right)}\right)=330 \mathrm{Hz}$

The difference of frequencies heard by the person$∆f=330-275=55 \mathrm{Hz}$