Given the speed and the distance, to find time you can use the formula speed is equal to distance over time. From there you can manipulate the equation for time to equal the distance divided by speed. Time is equal to 18.4 meters divided by 35m/s which equals 0.526 seconds.
A beat is an interference pattern between two sounds of slightly different frequencies, perceived as a periodic variation in volume whose rate is the difference of the two frequencies. Frequency beat is equal to,

The reference frequency in our case would be 392Hz, and since there is the possibility of the upper and lower range for the amount of beats per second that the two possible frequencies are heard would be


Therefore the two possible frequencies the piano wire is vibrating at, would be 396Hz and 388Hz
Velocity is a speed AND a direction.
When you turn a corner, or go around a curve in the road, your
direction changes, but the reading on the dial doesn't change.
So it can't be showing velocity. It must be showing only speed.
That's probably a big part of the reason why it's called a speedometer
and not a velocimeter.
Yes basic, calculus, and trig