Answer: 5/6
The complement of an event is basically the complete opposite.
If you define event A to be the event of selecting an R, then the complement of event A is not selecting R.
The probability of selecting R is 1/6 because there is 1 letter R out of 6 letters total. The complement of this is 5/6. You could count out 5 letters that aren't R, out of 6 total, or you can subtract 1/6 from 1 to get
1 - (1/6) = (6/6) - (1/6) = (6-1)/6 = 5/6
