1. **State the problem:** Maria buys computer disks at a price of 4 for 5 and sells them at a price of 3 for 4. We need to find how many disks she must sell to make a profit of 100. 2. **Identify the cost price (CP) and selling price (SP) per disk:** - Cost price per disk: $$\frac{5}{4}$$ - Selling price per disk: $$\frac{4}{3}$$ 3. **Calculate profit per disk:** $$\text{Profit per disk} = SP - CP = \frac{4}{3} - \frac{5}{4}$$ 4. **Find common denominator and subtract:** $$\frac{4}{3} - \frac{5}{4} = \frac{16}{12} - \frac{15}{12} = \frac{1}{12}$$ 5. **Set up equation for total profit:** Let $$x$$ be the number of disks sold. $$\text{Total profit} = x \times \frac{1}{12} = 100$$ 6. **Solve for $$x$$:** $$x = 100 \times 12 = 1200$$ 7. **Answer:** Maria must sell **1200** disks to make a profit of 100.