Convert 1-5 Grading Scale to 1-100 Grading System I am creating a formula in Excel to convert 1-5 Grading Scale to 1-100 Grading System Suppose that I have the following table:
97-100 = 1.00
94 - 96 = 1.25 
91-93 = 1.50 
88-90 = 1.75 
85-87 = 2.00 
82-84 = 2.25 
79-81 = 2.50 
76-78 = 2.75 
75 = 3.00
My problem is this: What if the grade is 1.47, what will be the exact numerical equivalent from 91-93? And how do I compute it?

I'm not quite sure I understand.  What it seems to me is that you have defined a function in the following way:
$$f: \{75,76,77,\dots,99,100\}\to \{1,1.25,1.5,\dots,2.75,3.0\} $$
$$f(x) = \begin{cases} 1~~~~~~~~~~~97\leq x\leq 100 \\1.25~~~~~~94\leq x\leq 96\\ 1.5~~~~~~~~91\leq x\leq 93\\ \vdots \\ 3~~~~~~~~~~~~x=75\end{cases}$$
As I understand, if you want to convert a score of $92$ from the grading scale out of 100 to the grading scale out of 5, it falls under the third case above, and $92\mapsto 1.5$.
Note a few things however: not only does $92\mapsto 1.5$, but also $93\mapsto 1.5$ as well as $91\mapsto 1.5$, a property we call "not being one-to-one".  In particular it means that $f^{-1}$ will not be a function.  That is to say, if we wanted to convert the number $1.5$ from the grading scale out of 5 to the grading scale out of 100, what number should it get?  Should $1.5\mapsto \begin{cases}91?\\92?\\93?\end{cases}$.
Furthermore, the value of $1.47$ does not appear in the range of $f$, so $f^{-1}(1.47)$ is not even defined.

Assuming you are willing to smooth the inverse function out by extending the domain (making it everywhere defined) and provide rules such as "if result is ambiguous, always go with the higher value" (making it well defined).
An example would be
$$f_*^{-1}(y) = \begin{cases} 100~~~~~~~~~~~1\leq y< 1.25 \\96~~~~~~~~~~~~~1.25\leq y < 1.5\\ 93~~~~~~~~~~~~~1.5\leq y<1.75\\ \vdots \\ 75~~~~~~~~~~~~~~y=3\end{cases}$$
Using this $f_*^{-1}$ (which is now a function) instead of $f^{-1}$ (which was not), we have that $f_*^{-1}(1.47) = 96$

One final note, perhaps an alternative approach would be to change the original $f$ so that $f$ would be one-to-one by making it so that it would map to a continuous line instead of a staircase function.
