Indirectly asked everything

Find the beat frequency detected by O ( $\in \text{Hz}$ ).The source of sound and observer move with velocities of $20\text{ m/s}$ and $40\text{ m/s}$ , respectively in the same line. The frequency of the source emitting the sound is $1000\text{ Hz}$ .

Note that a wall is placed in front of the observer.

Take velocity of sound in air $340\text{ m/s}$ .