They own a total of 120 CDs.
GIVEN:
Ella: ratio of rock music to total CDs that she owns 25/40
Paolo: has 50 rock music CDs. The ratio of rock music to total CD is equal to Ella's ratio.
Proportion: a/b = c/d where ad = bc
25/40 = 50/x
25x = 40 * 50
25x = 2000
25x / 25 = 2000 / 25
x = 80 total of Paolo's CDs
Total of Ella's CDs = 40
Total of Paolo's CDs = 80
Total CDs in all 120