Solve in English not French

The oscilloscope shows two waves. Channel 1 (speaker) represents the sound emitted, and Channel 2 (microphone) represents the sound received.
The time base is given as 0.5 ms/div.
By observing the graph, the peak of the wave from Channel 1 aligns with a major grid line. The corresponding peak for Channel 2 is 2 divisions to the right of the Channel 1 peak.
So, the time difference is:

$$\Delta t = 2 \text{ divisions} \times 0.5 \text{ ms/division}$$ $$\Delta t = 1.0 \text{ ms}$$

The distance $d$ is given as 0.85 m.
The formula for speed is:

$$v = \frac{d}{\Delta t}$$

$$v = \frac{0.85 \text{ m}}{1.0 \times 10^{-3} \text{ s}}$$ $$v = 850 \text{ m/s}$$

The speed of sound in air is $\boxed{\text{850 m/s}}$.