Keep assumptions short and decision-relevant. Explain the logic behind a key assumption, but do not narrate every obvious arithmetic step.

Casey responses benefit from visible reasoning, yet over-explaining can consume time that should go to calculation and interpretation.

State the assumption, one sentence on why it is reasonable, then calculate. Spend more explanation on uncertain or high-impact assumptions than on straightforward ones.

After calculating, interpret the result and connect it to the question. That usually demonstrates more judgment than additional pre-math commentary. For a related reference, see BCG Casey: Structuring Question Deep-Dive.

ngl this is one of those things that feels bigger in prep than it does in the actual interview. A simple response is usually enough.

What helped me was saying the assumption out loud in one sentence and then moving on. It made the reasoning easier to follow.