Answer:
19.54m
Explanation:
We can first calculate the minimum area for the sound to spread so that the librarian cannot hear them
[tex] A = \frac{1.2\times10^{-9}}{10^{-12}} = 1200 m^2[/tex]
For the sound to cover this area then the distance between the boy and the librarian must be
[tex]A = \pi r^2 = 1200[/tex]
[tex]r^2 = 1200 / \pi = 381.97 [/tex]
[tex]r = \sqrt{381.97} = 19.54 m[/tex]
