A violin player riding on a slow train plays

A violin player riding on a slow train plays a $440 \mathrm{~Hz}$ note. Another violin player standing near the track plays the same note. When the two are closed by and the train approaches the person on the ground, he hears $4.0$ beats per second. The speed of sound in air $=340 \mathrm{~m} \mathrm{~s}^{-1}$. (a) Calculate the speed of the train. (b) What beat frequency is heard by the player in the train?