I have a series of ratios that I need to combine by adding all the numerators
and then dividing by the sum of all the denominators. See below:

a/b c/d e/f sum=(a+c+e)/(b+d+f)

Unfortunately, the components are in single cells, e.g., cell A2 is "=a/b"
or else this would be an easy solution. (To make things worse, each
component (a, b, c, etc.) is actually a link to an external cell with the
actual data.) How can is extract the components in oder to perform the
operation? Thanks in advance.