Discussion
A single relationship connects every term shown here. Work out the rule and supply the missing number.
12, 21, 30, 39, ?
Answer: B
Correct answer: 48
Every gap is the same: the differences run 9, 9, 9, so the common difference is 9. Applying it once more to the final term, 39 + 9 gives 48. As a check, subtracting 9 from the answer returns 39, the term before it, and repeating that recovers the whole series. Where the difference is constant the series is an arithmetic progression, and its nth term can be written down directly rather than reached step by step.
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