TRAPDOOR =8 letters =8! =40,320 permutations. But, we have 2 repeated pairs [O,O and R,R], so we have a total of 8!/2!.2! =10,080 permutations. Each permutation begins with a letter, A,D,O,O,P,R,R,T. Each permutation that begins with any letter continues with that letter for: 10,080 / 8 =1,260 permutations. And since you have 4 letters, out of total 8 letters, repeated, the two "Os" and the two "Rs" are going to be adjacent to each other:1,260 x 4 =5040 times out of a total of 10,080 permutations, or 5040 /10,080 =1/2.This makes sense since 1/2 of the 8 letters are repeated.