How do we calculate bandwidth from signal formula?
Your intuition is basically correct.  I'm sure they want the answer of 80Hz because that is the bandwidth that you would need to carry these 5 waves, but pure sinusoids that aren't modulated at all can not carry any information.  If they don't carry any information then they are infinitely narrow.  (0Hz rather than 5Hz).  You could modulate them individually to higher frequencies and put them very close together with any loss of information.