Only 9 and 15 are odd so the rest can be rejected.
9 is not a triangular number so only 15 is odd and a triangular number.
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No. The first few triangular numbers are 1, 3, 6, 10, 15, 21, 28, ...
15 (1 + 2 + 3 + 4 + 5).
15 because its an odd number whereas 6, 10 & 12 are even numbers.
15
There are 10 odd number segments on the dart board: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 Some people also count the single bull (25) as an odd number.