You may have subtracted the midrange, 42, from the greatest known value in the set, 57, to get 15. This is not the missing value. If it were, it would be the least value. Then the midrange would be:

$\displaystyle\frac{{{15}+{57}}}{{2}}=\frac{{72}}{{2}}={36}\usepackage{cmbright}\Huge\$

instead of 42.