The five-second rule relates to how far away a thunderstorm is, in miles, for every five seconds between lightning and thunder.

• A. Ten seconds per mile
• B. One mile per five seconds
• C. Three miles per hour
• D. Five miles per second

Answer: (B)

Understanding how far away a thunderstorm is relies on how fast sound travels. Sound moves about 1 mile every 5 seconds, so the distance to the storm is roughly the time between the lightning flash and the thunder, in seconds, divided by 5. That makes the description “one mile per five seconds” the best way to express the rule. The other options don’t fit because they describe the relationship with the wrong units or a different speed concept: ten seconds per mile would invert the correct relationship, three miles per hour is a speed, not a distance-per-time rule, and five miles per second would imply an unrealistically fast sound travel rate.