IB Physics Doppler effect

A tutorial sheet of practice questions on the Doppler effect is given below.

1. A car moves at 20.0 m s^-1 to the east. The driver blows the horn of the car. If the speed of sound in air is 340 m s^-1, what is the speed of the sound waves that an observer measures if they are: (a) at rest to the west of the car, (b) at rest to the east of the car, (c) moving towards the car from the east at 20.0 m s^-1.
2. A car moves at a velocity u to the east. Sound waves of frequency f0 are made by the car. The sound waves reflect from a vertical wall and travel back to the car. What is the frequency of the sound waves received by the car? The speed of sound in air is c.