#### Answer

The rule is correct.

#### Work Step by Step

Light travels almost a million times faster than sound in air, so you essentially see the flash when it happens. It takes some time for the thunderclap to arrive at your ears.
The speed of sound in air is approximately $340 \frac{m}{s}$, and this is very close to $333 \frac{m}{s} = \frac{1}{3} \frac{km}{s}$.
Using the formula distance $d = speed \times time$, the distance in km from the storm is $\frac{1}{3} \frac{km}{s} \times (number \;of \;seconds)$.