#### Answer

a) 300 m/s.
b) 1 km every 3 seconds.

#### Work Step by Step

a. The rule of thumb estimates the speed of sound as 1 mile per 5 seconds. Convert:
$$\frac{1 mile}{5 s} \frac{1610 m}{mile}= 322 m/s$$
To one significant figure, that is $300 m/s$.
b. We see that sound would take roughly 3 seconds to travel about 1000 m, which is a km. The rule could be phrased as 1 km in 3 seconds.