Need help on EM waves problem

[Decan]

Homework Statement

An important news announcement is transmitted by radio waves to people who are 81 km away, sitting next to their radios, and by sound waves to people sitting across the newsroom, 2.3 m from the newscaster. Take the speed of sound in air to be 343 m/s. What is the difference in time that the message is recieved?

Homework Equations

c = f*lamda

The Attempt at a Solution

c1 = 3*10^8 m/s
lamda1 = 81km
c2 = 343 m/s
lamda2 = 2.3m

f1 = c1/lamda1
f2 = c2/lamda2

T = 1/(f1-f2) = time difference

What am I doing wrong? Thanks in advance for any help!

 

[21385]

I think this is more of a distance problem than a waves problem.

So, the speed of radio waves is the speed of light, 3*10^8 m/s...so use time=distance/speed
Do the same thing for sound, where it travels at 343 m/s.

 

[Decan]

awesome...thanks!