A 10 -year, $8 \%$ coupon bond currently sells for $\$ 90 .$ A 10 -year, $4 \%$ coupon bond currently sells for $\$ 80 .$ What is the 10 -year zero rate? (Hint: Consider taking a long position in two of the $4 \%$ coupon bonds and a short position in one of the $8 \%$ coupon bonds.)