You see lightning and 30 seconds later you hear thunder. how far away is the thunderstorm? take the speed of sound to be 339 m/s. g

Let the observer be 'd' distance away from the thunderstorm and let light take 't' time to reach the observer
Since the speed of sound and light remains constant in a particular medium, we can use
      Speed = Distance/Time

For light,
   3 x 10^8 = d/t
                t = d/(3 x 10^8)   -1 

For sound,
           339 = d/(t + 30)       -2

Putting value from 1 in 2.
               d = 10^4 m(approx)