Calculate Intensity of Sound Waves at 3.0 m from Source

[capudang]

Homework Statement

A small source emits sound waves with a power output 80W.
a) find the intensity at a point 3.0m from the source. (0.71W/m)
b) find the intensity level at the same point as part a.(118.51)
c) at what distance would the intensity be one-fourth as much as of that part (a).

i have no problem finding the answer for a and b, but i did have a bit of problem in c. i have tried to answer it which i get r=7.173, but i don't know is it right or wrong. help me for C.

2. Homework Equations c
B=10log(I/Io)
Ia=P/4pie r^2

The Attempt at a Solution

I=p/4pie R^2

i get 7.173 after calculate it.

 

[DEvens]

Check your arithmetic. How much is pi? How much is R? How much is 4 pi R^2?