college physics

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A 55-kg woman cheats on her diet and eats a 465-Calorie (465 kcal) jelly doughnut for breakfast.
(c) If the human body is only 24% efficient in converting chemical energy to mechanical energy, how many stairs must the woman climb to work off her breakfast?

I got 24075.32468 stairs to perform an amount of mechanical work equivalent to the food in one jelly doughnut (which is correct). I assumed that I would just multiply that number by 0.76 (100%-24%) and then add my answer to 24075 stairs, but that was wrong. I then tried the 24% but that was wrong to. What do I do?

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