Rolex buys 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

Given that Rolex buys computer disks at a price of 4 for $5 .That means cost price of 1 disk = 5/4
Given that Rolex sells them at a price of 3 for $5.That means selling price of 1 disk = 5/3
Profit on 1 disk = selling price - cost price Profit on 1 disk [tex] = \frac{5}{3}-\frac{5}{4} [/tex]Profit on 1 disk [tex] = \frac{5}{3}\cdot\frac{4}{4}-\frac{5}{4}\cdot\frac{3}{3} [/tex]Profit on 1 disk [tex] = \frac{20}{12}-\frac{15}{12} [/tex]Profit on 1 disk [tex] = \frac{5}{12} [/tex]Now we have to find how many disk to be sold to get the profit $100so we just divide $100 by profit on 1 disk[tex] \frac{100}{\left(\frac{5}{12}\right)} = 100\cdot\frac{12}{5} = 240 [/tex]Hence final answer is 240 disks.