Final answer:
The half-life of erbium-165 is needed to determine the amount that was radioactive 24 hours ago.
Step-by-step explanation:
The question asks for the amount of erbium-165 that was radioactive 24 hours ago, given that 5.0 grams are currently radioactive. To answer this question, we need to know the half-life of erbium-165. Unfortunately, the provided information does not include the half-life of erbium-165, so we cannot determine the amount that was radioactive 24 hours ago without this information.