How many brain coral colonies did the biologist count?

Question

During a coral reef survey, a marine biologist counted 60 coral colonies along a transect, each classified as either staghorn coral or brain coral. The number of staghorn colonies was 12 more than twice the number of brain coral colonies.

Accepted Answer

A. 16. Let b be brain coral colonies and s staghorn colonies. Then b + s = 60 and s = 2b + 12. Substituting: b + (2b + 12) = 60, so 3b = 48 and b = 16 (and s = 44, which checks: 16 + 44 = 60). Choice B comes from dropping the '12 more' and solving b + 2b = 60, giving b = 20. Choice C comes from a sign error, moving the 12 to the wrong side: 3b = 60 + 12, so b = 24. Choice D is the number of staghorn colonies, the other variable, not the one asked for.

Answer

B. 20 — You dropped the '12 more' and solved b + 2b = 60; the staghorn count is 2b + 12, so the equation is 3b + 12 = 60.

Answer

C. 24 — You moved the 12 to the wrong side, solving 3b = 60 + 12 instead of 3b = 60 - 12; subtracting gives 3b = 48 and b = 16.

Answer

D. 44 — 44 is the number of staghorn colonies — the other variable. The question asks for brain coral colonies, which is 16.

Algebra practice question

Answer: A. 16

Why the other answers are wrong

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