What equal annual series of payments must be paid into a sinking fund to accumulate the following amounts?

(a) $45,000 in 11 years at 8% compounded annually.
(b) $35,000 in 18 years at 6% compounded annually.
(C) $25,000 in 6 years at 7% compounded annually.
(d) $12,000 in 13 years at 11% compounded annually.