Yearly market for replacement car batteries

Estimate how many car batteries are sold in the US each year.

Roughly 70 to 100 million a year. There are about $280$ million vehicles on the road; a battery lasts around $4$–$5$ years, so replacements are about $280/4.5\approx60$ million a year. Add about $15$ million batteries fitted to new vehicles, plus boats, motorcycles and early failures, giving roughly $75$–$90$ million. Spell out each assumption so the interviewer can follow and adjust it.