There are six possibilities to take out two numbers (1 2), (1 3), (1 4), (2,3), (2,4) (3,4).

There are five possibilities for writing X: 3, 4, 5, 6, 7;

Probability p (x = 3) =1/6;

p(x = 4)= 1/6；

p(x = 5)= 2/6 = 1/3；

p(x = 6)= 1/6；

p(x = 7)= 1/6；

Expected ex = 3 *1/6+4 *1/6+5 *1/3+6 *1/6 = 5;