When a train is approaching the observer, the frequency of the whistle is 100 cps. When it has passed the observer, it is 50 cps. The frequency when the observer moves with the train, is :

Explanation

According to Doppler effect in case of approachign of source, `f. = f v/(v -v_s) , i.e., 100 = (fv)/(v -v_s)`
`i.e., v -v_s = (fv)/(100)`, while in case of recession of source.
`f. = f v/(v +v_s) i.e., 50 = (fv)/(v +v_s) , v +v_s =(fv)/50`
So adding the tow eqns., `2V =(3fv)/100`
`i.e., f =200/3 = 66.67 Hz`