While it may appear that the odds of matching the Mega Ball should be 1 in 15 (as there are only 15 numbers possible to match), that is not the case. The odds of matching ONLY the Mega Ball number is a combination of two probabilities:
1. Not matching any white ball numbers, and,
2. Matching only the Mega Ball.
When the math equations for both those probabilities are completed, the odds for matching the only the Mega Ball are 1 in 21.