The equation we have to solve is:

Steps to solve the equation for x.
Step 1. Use distributive property on the left side to multiply 25 by x and by -4:

That is because 25 by x is 25x and 25 by -4 is -100.
Step 2. Subtract 25x from both sides:

Step 3. Divide both sides by 5:

Since x is negative, that means he needs to make 20 sales to break even.
Answer: 20 sales.