AHSME 1999 Problem 3

The number halfway between $1/8$ and $1/10$ is $\mathrm{(A) \ } \frac 1{80} \mathrm{(B) \ } \frac 1{40} \mathrm{(C) \ } \frac 1{18} \mathrm{(D) \ } \frac 1{9} \mathrm{(E) \ } \frac 9{80}$