Company A charges a flat fee of 60 plus 2 per unit produced. Company B charges a flat fee of 20 plus 4 per unit produced. At how many…

Write each company's total cost for $n$ units: Company A is $60 + 2n$ and Company B is $20 + 4n$. The costs match when $60 + 2n = 20 + 4n$. Subtract $2n$ and 20 from both sides: $40 = 2n$, so $n = 20$, choice D. Check: A charges $60 + 40 = \$100$ and B charges $20 + 80 = \$100$. Choice A (15) and choice C (10) give different totals. Choice B (40) is the difference in flat fees, which is a value on the way to the answer, not the final step. Divide by the difference in per-unit rates.