What equal series of payments must be paid into a sinking fund to accumulate the following amount? (a) $21,000 in 10 years at 6.45% compounded semiannually when payments are semiannual. (b) $9,000 in...

(a) $21,000 in 10 years at 6.45% compounded semiannually when payments are semiannual.

(b) $9,000 in 15 years at 9.35% compounded quarterly when payments are quarterly.

(c) $24,000 in 5 years at 6.55% compounded monthly when payments are monthly.