During a Profitability question on whether or not a fast food rewards program would be profitable, I first correctly calculated the revenue increase based on additional visitors. I then used this number of additional visitors to calculate variable costs, but should have used the total number of visitors (past + additional).
The interviewer asked mid-calculation if I was accounting for all visitors and I correctly fixed my math, though I was wondering if this would be enough to reject me assuming every other aspect of my case was fine. What if the interviewer didn't point this out?