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

Explanation:
In the first line: +2, +4, +8. In the second line: +3, +9, +27. In the third line: +4, +16, +64.
(11 + 16 = 27).