Civil Service Exam Reviewer · Number Series · hard

What number(s) should complete the series: 3, 9, 5, 25, ?

1. 625
2. 29
3. 17
4. 21

Answer

D. 21

Solution

Alternately square and subtract 4: 3² = 9, −4 → 5, ² → 25, −4 → 21.

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