Speed of sound on a moving train

1. The problem statement, all variables and given/known data

Two trains A and B are moving with speed 20m/s and 30m/s respectively in the same direction on the same straight track with B ahead of A. The engines are at the front ends. The engine of trains A blows a long whistle.

Assume that the speed of sound of whistle is composed of components varying in frequency from f1=800Hz to f2=1120 Hz as shown in the figure. The spread in the frequency (ie, highest frequency – lowest frequency) is thus 320 Hz. The speed of sound in air is 340m/s.

Find the speed of the whistle for passengers in A and passengers in B

2. Relevant equations

3. The attempt at a solution

Relative velocity between A and B=10m/s. So speed of sound for passengers in B would be 10m/s less than 340m/s i.e, 330m/s and for people in A the speed of sound would be 340m/s.
But my answer is wrong. The answer is 360m/s for people in A and 310m/s for people in B. I could not think of a possible explanation. Can anyone help?

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