To the nearest second, how many seconds elapse between when a flash of lightning is seen and when the clap of thunder is heard if the lightning
a. a kilometer away?
b. 1 mile away (1.6 km away)?

a. 2.9 s

First of all, we can assume that the light of the lightning reaches us instantaneously, since the speed of light is very large and so light takes a negligible time to reach us.

Secondly, the speed of sound in air is

So, the difference in time between the moment we see the flash of the lightning and the moment we hear the thunder is simply given by the time it takes for the sound wave to cover the distance of  d = 1 km = 1000 m, so:

b. 4.7 s

This part of the exercise is exactly identical to the previous one, except for the fact that this time the distance that the sound should cover is

d = 1.6 km = 1600 m

Therefore, the time difference is

well clustered together.

points close on a scatter diagram