What number would replace the question mark in the series given below? 1, 5, 11, 19, 29, 41, 55, ?

The next number is 71.

The differences increase by 2 each time: 4, 6, 8, 10, 12, 14.

1+4=5, 5+6=11, 11+8=19, 19+10=29, 29+12=41, 41+14=55.

The next difference is 16.

55 + 16 = 71.

Method: second differences are constant at 2.