<span>gimme a second i a thinking.
</span>
<span>Rayed craters-</span><span> were the last features to form on the moon.</span>
We know that frequency is an inverse value of time
this implies that time is the inverse value of frequency
.
Now since <em>f</em> is <em>40kHz</em><em>,</em><em> </em>we can calculate period or duration for that matter time.

Hope this helps.
r3t40
When the magnet is near, the compass turns and points to it. When the magnet is moved away, the compass turns back to point north and south.