Answer
300 m
Work Step by Step
The distance traveled is the same, and equals vt. The sound travels faster in concrete and takes less time. The speed of sound in concrete is given to one significant figure in Table 12-1.
$$D=v_{concrete}t_{concrete}=v_{air}t_{air}$$
$$(3000m/s)(t_{air}-0.80s)= (343m/s)(t_{air})$$
Solve for the time the sound traveled in air.
$$t_{air}=\frac{3000m/s}{3000m/s-343m/s}(0.80s)$$
The desired distance D is $v_{air}=343m/s$ multiplied by that.
Putting in numbers, $D \approx 300m$.