Even if this is a homework assignment as XpUser comments, it is impossible to answer your question from the information provided.
You have a 15 cell range (C1:C15) and you state that what you have entered into this range is 20' & 40'
Is that one text string "20' & 40'" in all 15 cells or is it 20' in C1 and 40' in C15.
What is 'as same'
Does this apply to the 20' & 40' or is it to do with the range F1:F15
What is in the cells F1:F15
Are some of the cells empty and some contain "ldd", or do some cells contain the word "empty"
What is the meaning of "Idd". Is it an abbreviation for something and does it have any relevance for the solution you are looking for.
Finally what does
i want to find the real value of "20' empty"
Are you looking for a formula to extract the number 20 from it.
So in summary, you need to be specific about what has been entered into each cell in the ranges you mention, and you have to state what output you are looking for, in a way that shows the relationship of the output to the data in the cells.
I also note that you have posted in the XP forum. The Office Software forum is a more appropriate place to post, as that forum deals with Excel issues.