Rolex buys a computer disks at a price of 4 for $5 and sells them at a price of 3 for $5 . how many computer disks must he sell to make a profit of $100

He buys them at a unit price of 0.80 a CD. (to find this just divide the number of materials by the cost) And he sells them for 0.60 a piece. Then, you have to divide 100 by 0.60, which would be estimated to around 167. ( the exact number is 166.6666667) So, about 167 CD's would have to be sold by him to make at least a profit of $100. 
Hope this helped.