Discussion
One term of this sequence has been replaced by a question mark. Work out the rule and supply the missing number.
5, 15, 45, 135, ?
Answer: C
Correct answer: 405
The terms are multiplied rather than added to. Dividing each by its predecessor gives the same result throughout: 15 over 5 is 3, and 45 over 15 is 3, so the ratio is 3. Multiplying the last term, 135 times 3 is 405. As a check, dividing the answer by 3 returns 135, which is the last term shown. A single pair of terms is consistent with both an addition and a multiplication rule, so at least two pairs must agree before a ratio can be trusted.
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