If David has 83 CD's, and each rack holds 25, then David would need 4 racks. 4 racks will hold up to 100 CD's, where as 3 racks would only hold 75.
I'm not sure what 'Lin' has to do with this equation, but if she has 6 racks full of CD's then in total she has 150 CD's.
Hope this helps! :)
