Easy wave group question

[timhunderwood]

1. The problem statement, all variables and given/known data
A wave group is formed by modulating the amplitude of an underlying wave at 100rad s^-1 as shown:

http://img189.imageshack.us/img189/162/qb7.png

Estimate the range of frequencies present in this pulse.

3. The attempt at a solution

I'm confused as to how to start could someone point me in the right direction?

Thanks