Here's the text of a Denver, CO, Craigslist ad:
550-600 CD'S - $200 (Aurora) Great collection of music all for $250. No cases all in great shape. 

I see 7 stacks of 50 CDs, a stack of 25, and a stack of 100.

7 x 50 = 350 + 25 + 100 = 475.

I know I'm not good in math, but there's a difference of 75 between 550 and 475. Interesting.