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I was trying this 3,9,18,______,648 What is the missing number? Any hints?

I am given options as:

  1. 27
  2. 324
  3. 54
  4. 108

I thought of adding the digits and then see a pattern like except 3 all the digits of the numbers add to 9, but that is for all the numbers given in the options.

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Perhaps square, double, square, double:
$3^2=9$
$9\cdot2=18$
$18^2=\boxed{324}$
$324\cdot2=648$

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  • $\begingroup$ Nice observation!!! $\endgroup$ – BAYMAX Jun 3 '18 at 9:36

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