Velocity of lightning

[timtng]

A common method for estimating the distance to
a lightning flash is to begin counting when the flash is
observed and continue until the thunder clap is heard.
The number of seconds counted is then divided by 3 to
get the distance in kilometers. What is the velocity
of sound in kilometers per second?
(The speed of light is 3*10^8 m/s.)

I have no idea how to start this problem, please help?

[Doc Al]

For all practical purposes, you can assume that you see the lightning with no delay. The sound of the thunder is delayed due to the much slower speed of sound.

Use Distance = Speed x Time to figure this one out.