Answer
It would take $15$ seconds for the sound of thunder to reach us.
Work Step by Step
We are $3$ miles away from the lightning flash, and we want to know how long it would take us to hear the sound of thunder.
We use the formula given, so we substitute $3$ for $M$, the distance we are from the lightning flash. We get:
$3 = \frac{n}{5}$
To solve for $n$, the time in seconds it would take for the sound of thunder to reach us, we multiply both sides by $5$ to get rid of the fraction:
$n = (3)(5)$
$n = 15$
