To find the theoretical probability that Nadir will be chosen, you need to find the number of favorable outcomes and divide that by the number of total outcomes. I'm assuming you learned permutations so to find the number of total outcomes, you do 6P4 (number of ways to seat 4 people out of the six students where order matters). However, the positions are identical which means order does not matter so you have to take 6C4 to find the favorable outcomes (probability that nadir will be chosen as part of the group. Therefore, to find the theoretical probability, you do 6C4 / 6P4.
To find 6C4 the formula is n! / (n-r)!r! so 6! / (6-4)!4! = 6*5*4*3*2*1 / 2*1*4*3*2*1 = 6*5 / 2 * 1 = 15 To find 6P4 the formula is just n! / (n-r)! --> 6! / 2! --> 6*5*4*3*2*1 / 2*1 = 360 15/360 = 1/24
