The apparent frequency of the whistle of an engine changes in the ratio 9:8 as the engine passes a stationary observer. If the velocity of the sound is 340 ms^-1, then the velocity of the engine is

When the engine approaches the observer, the apparent frequency is v/(v – v_S) and when it moves away from the observer, the apparent frequency is v/(v + v_S). The frequency therefore changes in the ratio (v + v_S)/(v - v_S). We have therefore, (v + v_S) /(v- v_S) =9/8 from which the velocity of the source, v_S = v/17 = 340/17 = 20 m/s.