Formula SUM - exclude cells with text / value problem

Hi

Is there a way to do a Formula (SUM) but exclude cells with invalid values?
I would like to add up all values in attached spreadsheet for cells A14 to A28 and again for A49 to A63, as well as get an average C-14 to 28 and again for C49-63.

As some of the cells contain text rather than numbers, the formula doesn't work. Any idea how to exclude these cells?

Thanks

Renix