To convert the number stored as text to an actual number, choose any empty cell and apply this formula: =N1*1. The N1 will be the cell where the number stored as text appears. For example, if the text I want to convert to a number appears in cell E5 (column E, row 5), my formula would be =E5*1.
Once you use that formula to recalculate the original cell, copy the formula for additional cells as required.
Select the cells where you performed the calculation and copy them.
In the original cells (E5 in our example above), right click and choose "Paste Special". In the paste special box, select Values, to paste the calcluated amount.
Your numbers will now appear as numbers and you can run formulas on them.