Sound moves with higher velocity if

A wall is moving with velocity u and a source of sound moves with velocity (u)/(2) in the same direction as shown in the figure. Assuming that the sound travels with velocity 10u , the ratio of incident sound wavelength on the wall to the reflected sound wavelength by the wall is equal to

A wall is moving with velocity u and a source of sound moves with velocity (u)/(2) in the same direction as shown in the figure. Assuming that the sound travels with velocity 10u , the ratio of incident sound wavelength on the wall to the reflected sound wavelength by the wall is equal to

Can we apply Doppler effect to a source of sound moving faster than the velocity of sound?

An observer moves towards a stationary source of sound with a velocity one-fifth of the velocity of sound. Calculate the percentage increase in the apparent frequency?