55 is the odd one out because it is not divisible by 10. 10, 20, and 30 are all multiples of 10 (10 × 1, 10 × 2, and 10 × 3 respectively). 55 is a multiple of 5 but not of 10, as it does not end in 0. Additionally, 55 is not part of the arithmetic sequence 10, 20, 30 where the common difference is 10.