Using sum function where cell contains a formula

SUM doesn't care if values are the results of formulas or not, it sums all numbers regardless

If you are having problems summing "numbers" returned by formulas it may be that they are text formatted.

If the formula is returning a number in quotes like "23" or "158" then those are text values and SUM function will ignore them - if you want them to be treated as numbers then remove the quotes

Values returned by LEFT, RIGHT, MID and other functions are, by defualt, text values, e.g. if A1 = xyz123 and you use this formula in B1

=RIGHT(A1,3)

The value returned is a text value - you can do something like this to make it in to a number

=RIGHT(A1,3)+0

If those suggestions don't help then please give some examples of the formulas used, where the results are not summed

yes it is! but what do you actually have in those cells that stops you doing it ?

Let me guess...

You're trying to use a SUM function on a range of cells that contain formulas and it's returning 0?

In your formulas remove any quotes from around numbers: "10". Quoting numbers turns them into TEXT strings and the SUM function ignores TEXT values.