Another way is to enter this array* formula instead of your normal sum(
) formula:

=SUM(VALUE(range of cells))

*As this is an array formula, once you have typed it in (or
subsequently edit it) you must use CTRL-SHIFT-ENTER instead of the
usual ENTER. If you do this correctly then Excel will add curly braces
{ } around the formula - you must not type these yourself.

Though changing the text to numbers as described above is the better
way if you want to use the numbers in other calculations, this gives
you an alternative.

Hope this helps.

Pete