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18. What figure should replace the question mark?
  IQ test. Free iq test online # 1 with answers and solutions. Question # 18. What figure should replace the question mark?
 

Explanation:
First line: +2, +4, +8 (2 raised to (1, 2, 3) power). Second line: +3, +9, +27 (3 raised to (1, 2, 3) power). Third line: +4, +16, +64 (4 raised to (1, 2, 3) power).