Suppose that annually an average of library patrons want to borrow a book. A patron borrows

Suppose that annually an average of λ library patrons want to borrow a book. A patron borrows the book for an average of 1/µ years. Suppose we observe that the book is actually borrowed an average of R times per year. Explain how we can estimate λ, which is an un-observable quantity. (Let U be the expected number of times per year a patron wants to borrow the book and the book is out. Note that λ = R + U.)