Waves

The frequency of sound heard by an observer moving towards a stationary source with certain speed is $n_1$ and if the observer moves away from the same source with same speed, the frequency of sound heard by the observer is $n_2$. If the speed of sound in air is $340 \mathrm{~ms}^{-1}$ and $n_1: n_2=71: 65$, then speed of observer is