Reduction in channel bandwidth calculations

Hello all,

I am tasked with finding the reduction in channel bandwidth requirement for iLBC 20ms and 30ms blocks.

I have calculated this to be 12Hz (find the frequency for both blocks and subtract), however i am unsure if this is the solution.

Are they asking me for a reduction in channel bandwidth from input to output?

Can someone point me in a direction or give me an idea of how to solve this?

iLBC specifications can be found: https://en.wikipedia.org/wiki/Internet_Low_Bitrate_Codec

Thanks.