are you sure with the ans? i don't think so it is right.

 

if deector is moving in +ve x-axis direction &soruce in -ve x-axis direction

then

                                                                  for both V_S &V_d    =a*t=20

       fs/V-Vs =(f)_d /V+V_d

so f_d=f_s(330+V_d) / 330-V_s

 

     f_d=330+20 / 330-20

f_d=35/31 khz