Answer: 100 of the CDs are single album and 25 are double album.
Explanation: What you have to do first is find out how many single CDs which are .5 cm in length you can put in 125 with out going over 75 cm. Then the rest of the 25 are of course double album CDs and which equal out to be 125 CDs total and 75 cm total. (the double album CDs technically count asone by the way) I got my answer by first seeing how many double CDs can go in 125 without going over 75 cm. The max you can get is 75. So then you keep removing double album CDs until you get the magic number 125 CDs and 75 cm.
Hope I could help