First convert the speed of sound from meters/second to miles/second:
[tex]340\,\dfrac{\mathrm m}{\mathrm s}\cdot\dfrac{3.281\,\mathrm{ft}}{1\,\mathrm m}\cdot\dfrac{1\,\mathrm{mi}}{5280\,\mathrm{ft}}=0.21\,\dfrac{\mathrm{mi}}{\mathrm s}[/tex]
Then the time it takes for the sound to travel 20 miles is
[tex]\dfrac{20\,\mathrm{mi}}{0.21\,\frac{\mathrm{mi}}{\mathrm s}}=95\,\mathrm s[/tex]
