There are 3 D's in "Dungeons and Dragons," which is 18 letters. But if it's 11.1% to you, that's a valid option.66.7% of "D&D" and 11.1% of "Dungeons and Dragons" - I'm not sure where your figure of 16.7% comes from.
Johnathan
This one took me a minuteIt also depends on the edition.
1st, 4th and 5th edition have less 'D' than 2nd and 3rd Edition.