Need help...recursion/finite sets

PLz help me do these problems...thank youuu sooo much.

1) An amortized loan is paid off in equal periodic payments of P units. Assume that the periodic interest rate over the time period is (100 X r)%. If L is the loan amount and Xn is the balance remaining after the nth payment, give a recursive formula for Xn.

comment: usually one is given L and r, and the objective is to find a value of P such that the balance is equal to zero after the Mth payment for some fixed M (which is normally a multiple of 12). In order to do this, one needs to derive a closed formula for Xn and solve for P in terms of L, r, and M.

Finite Sets:

1) Determine the number of Boolean subalgebras of P(X) if X is the set {1,2,3,4}. How many have exactly two atomic subsets?