HSC Physics Doppler effect

A tutorial sheet on the Doppler effect for sound is given below.

1. The speed of sound in air is 343 m s^-1. A train moves towards a stationary observer at 100 m s^-1. What is the speed of the sound waves made by the train relative to the observer?
2. In question 1 the train has a horn that emits sound waves of frequency 200 Hz. Find the frequency of these sound waves that is measured by a stationary observer in front of the train.
3. In question 2 find the wavelength of the sound waves in front of the train.
4. The sound waves reflect back towards the train from a vertical mountain in front of the train. Find the frequency of the sound waves arriving back at the train.
5. A second train moves at 50 m/s towards the first train. Find the frequency of the sound waves that reflect off the second train and arrive back at the first train.
6. A stationary observer is at a perpendicular distance of 50 m from the straight train line. Assuming that the first train moves at a constant speed, find the maximum and minimum frequencies detected by the observer as the train approaches and recedes.