(Related to Checkpoint 5.6) (Solving for ) Springfield Learning sold zero coupon bonds (bonds that don't pay any interest, instead the bondholder gets just one payment, coming when the bond matures, from the issuer) and received $900 for each bond that will pay $18,000 when it matures in 35 years. a. At what rate is Springfield Learning borrowing the money from investors? b. If Nancy Muntz purchased a bond at the offening for $900 and sold it 20 years later for the market price of $3,300, what annual rate of return did she earn? c. If Barney Gumble purchased Muntz's bond at the market price (S3,300) and held it 15 years until maturity, what annual rate of return would he have earned?