Let me give this a crack:
=(SUM(B2:2)-MAX(B2:2))/2 + MAX((B2:2))
=(SUM(B2:2)-MAX(B2:2))/2 + MAX((B2:2))
Presuming all the data is in the one row, the above should work, just drag it down. Should be easier to understand too :).
The idea is to sum the whole row, subtract the largest number, divide that total by two then add the largest number back in.