A shelf has 2 math books and 3 physics books. Two of the books are selected at random. Let X be the number of math books in the sample. Construct a probability table for X. Find E(X) using the table.

$$P(X=j)=\binom{5}{2}(2/5)^{j}(3/5)^{j-1}$$
$$P(X=2)=\binom{5}{2}(2/5)^{2}(3/5)^{0}=0.16$$
$$P(X=1)=\binom{5}{2}(2/5)^{1}(3/5)^{1}=0.48$$
$$P(X=0)=\binom{5}{2}(2/5)^{0}(3/5)^{2}=0.36$$

$$E(X)=np$$

I don't really understand what E(x) is/how to compute it?