I'm going to say 30, because it has three prime factors (2 x 3 x 5) while the other numbers have two prime factors: (10 = 2 x 5, and 55 = 11 x 5).
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55 has no 3 factor
55, the rest are divisible by 3
The tenth triangular number is equal to (10 x 11) / 2 = 55.
A Kaprekar number is a number whose square can be split into two parts whose sum is the the original number. Both parts must be positive.For example, 55^2 = 3025 and 30+25 = 55.This is an example in base 10. Kaprekar also defined his number for other bases.
55 = (5 x 10) + (5 x 1)