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 $ $1000$ for each bond that will pay $$18000$ when it matures in $40$ years. purchased a bond at the offering for $$1000$ and sold it $10$ years later for the market price of FV Subscript n$,$ what annual rate of return did she earn?