Question

I need two equations to solve for the number of songs and pictures. I just need it straight forward no need to explain it.

Answer 1

you can proceed like this,

650 kb for a pic
and 2200kb for song ( 650 + 1550)

total= 18150 kb  and 16 files

so, 650 kb each in both pic and song = 16 * 650= 10400

remaining = 18150-10400 = 7750 , which is contributed by  additional 1550 of songs that is  
7750/1550 = 5

so, a total of 5 songs and 11 pictures were  sold..

hope it helped