Physics Question on doppler effect

Question

speeds of $20\, m/s$ and $15\, m/s$ relative to the ground. The first train sounds a whistle of frequency $600\, Hz$ . The frequency of the whistle heard by a passenger in the second train before the train meets, is (the speed of sound in air is 340 m/s)

Answer 1

Solution

The frequency of whistle heard by passenger in the train B, is
$n =\left(\frac{ v + v _{0}}{ v - v _{ s }}\right)$
$=600\left(\frac{340+15}{340-20}\right) \approx 666\, Hz$