You may have added the midrange, 42, to the least known value in the set, 23, to get 65. This is not the missing value. If it were, it would be the greatest value. Then the midrange would be:

$\displaystyle\frac{{{23}+{65}}}{{2}}=\frac{{88}}{{2}}={44}\usepackage{cmbright}\Huge\$

instead of 42.