what does this question mean?

(SAMC). The Hamming distance H($1, $2 ) between two bit strings $1 and $2 of equal length is the number of positions at which the corresponding bits are different. For example: H(001, 100) = 2 H(1101, 1001) = 1 Consider randomly selecting two bit strings $1 and $2 from the set of all bit strings of length 5 with repetition (i.e. it is allowed that the two strings are equal). Find E( H($1, $2) ), the expected value of the Hamming distance between $1 and $2. 0 5 C(5,1)+2.C(5,2)+3.C(5,3)+ 4.C(5,4) +5.C(5,5) 25 -1 O 15 O O N/CT