According to a rule-of-thumb, every 5 seconds, between a lightning flash and the following thunder gives the distance to the flash in miles. assuming that the flash of light arrives in essentially no time at all, estimate the speed of sound in m/s from this rule.

Given the speed of the sound in the problem which is 1 mile per 5 seconds. 

The speed is calculated by:

Speed = distance/time = (1mi/5s) (1610 m/1mi) = 300 m/s

Note that only 1 significant figure is given which is 5 second and so only 1 significant figure is justified in the result. The speed of sound is 343 m/s. therefore the rule of thumb is fairly close.