An issue of bonds, redeemable at par in n years, is to bear coupons at 9% com- pounded semi-annually. An investor offers to buy the entire issue at a premium of 15%. At the same time, the investor advises that if the coupon rate were raised to 10% compounded semi-annually, he would offer to buy the whole issue at a premium of 25%. At what yield rate compounded semi-annually are these two offers equivalent?