Consider three sets E
1 = {1, 2, 3}, F
1 = {1, 3, 4} and G
1 = {2, 3, 4, 5}. Two elements are chosen at random, without replacement, from the set E
1, and let S
1 denote the set of these chosen elements. Let E
2 = E
1 S
1 and F
2 = F
1 S
1. Now two elements are chosen at random, without replacement, from the set F
2 and let S
2 denote the set of these chosen elements.
Let G
2 = G
1 S
2. Finally, two elements are chosen at random, without replacement, from the set G
2 and let S
3 denote the set of these chosen elements.
Let E
3 = E
2 S
3. Given that E
1 = E
3, let p be the conditional probability of the event S
1 = {1, 2}. Then the value of p is