Each number is twice the preceding one with 1 added or subtracted alternately.
So, the next number is (2 x 261 + 1) = 523.
There are two series, beginning respectively with 3 and 7. In one 3 is added and in another 2 is subtracted. The next number is 1 - 2 = - 1.
Each number is double the preceding one plus 1. So, the next number is (255 x 2) + 1 = 511.
Number are alternately multiplied by 3 and divided by 2. So, next number = 54 / 2 = 27.
There are two series (8, 11, 14, 17, 20) and (7, 12, 17, 22). Increasing by 3 and 5 respectively.
Subtract 20, 25, 30, 35, 40, 45 from successive numbers. So 0 is wrong.
Go on subtracting 3 and dividing the result by 2 to obtain the next number. Clearly, 46 is wrong.
Go on adding 7, 9, 11, 13, 15, 17 respectively to obtain on the next number. So, 33 is wrong.
Go on subtracting 24, 21, 18, 15, 12, 9 from the numbers to get the next number. Clearly,128 is wrong.
Numbers are (23 - 1), (33 - 1), (43 - 1) (53 - 1), (63 - 1) (73 - 1) etc.
So, the next number is (83 - 1) = (512 - 1) = 511.
In each number except 427, the middle digit is sum of the other two.
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