what is the 20 th no. of this series 3,18,52,132.............

We don't do homework questions, and certainly not ASAP.

However, take 3, double it and add 12 to give 18
18, doubled add 16 (four more than 12) to give 52
Double it, add 20 (four more than 16) to give 132
Continue along that line to yield 10485668 as the 20th iteration.

Assuming that the iteration jumps are correct.

[From WW]
I'm trying to follow your math, but it doesn't add up.
3*2 + 12 = 18 *correct
18*2 + 16 = 52 *correct
52*2 + 20 = 124 *incorrect

The 20th iteration of that would be 9961380.

He posted the question again and someone found: http://www.research.att.com/~njas/sequences/A061317[^] which shows a solution.