These are all possible seating arrangements with A and B in the end seats: {ACDB,ADCB,BCDA,BDCA} That's 4 ways out of 4!=24. So the probability is 4/24 which reduces to 1/6. Edwin
The number of all possible permutations of 4 persons is 4! = 4*3*2*1 = 24. The number of all possible arrangements, where A an B occupy the end seats is 2*2 = 4 (two positions for A abd B; and two positions for C and D). Therefore, the probability is P == .