Expectation Value of Nsub.1 for Rare Species: What is <N> & DeltaN?

[mna skt]

the number of hairs Nsub.1 on a certain rare species can only be the number 2sup.l(l=0,1,2...) The probability of finding such an animal with 2sup.l hairs is exp-1/l ! what is the expectation,<N>? what is deltaN?

 

[mna skt]

i have solved that question but could anybody please tell me some thing about how to grade the concept and some thing about tensor in more easy way