A person observes two moving trains, A reaching the station and B...

Using Doppler effect, apparent frequency is given by $f = \left(\dfrac{v \pm {v}_{0}}{v ∓ {v}_{s}}\right){f}_{0}$, where ${f}_{0} = \text{actual frequency}, {v}_{0} = \text{velocity\text{ of } observer and}{v}_{s} = \text{velocity of source}$

For train $B$ leaving the station,

${f}_{1} = 300\left(\dfrac{330 - 0}{330 - \left(- 30\right)}\right) = 275Hz \left(∵ {v}_{0} = 0\right)$

For train $A$ reaching the station,

${f}_{2} = 300\left(\dfrac{330 - 0}{330 - \left(30\right)}\right) = 330Hz$

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