im new - how to do this?

sorry im new and i dont know how to do this.
i have 2 mp3 files (a.mp3 and b.mp3) opened in audicity, both are 1 minute long.

what i would like to do is play b.mp3 5 seconds before a.mp3 ends. i have moved b.mp3 to the 55 second using the time shift tool but my problem is at second 55.
b.mp3 starts to play but i cant hear it clearly because a.mp3 is also playing. (like when 2 people talk you cant understand anything)
what i would like to do is play b.mp3 louder and a.mp3 last 5 seconds play softer like in the background that way b.mp3 can be heard.
i have tried the effects (cross / fade in, fade out) but no help

how can i do this?
thank you

You could use the “Envelope Tool” to adjust the volume: http://audacityteam.org/manual/index.php?title=Envelope_Tool