Auto calculate to find number needed for a percent

[Alindamood]

Hello! I need some help please!
Trying to find a way to auto calculate what is needed to get a certain percentage.
For example:
If I have 50 of 100 (50%) how do I auto calculate what is needed to reach 80%
I feel like this is easy and I am over thinking this.

[#2]

@Alindamood 

Let's say 100 is in A2, the current value 50 is in B2, and the desired percentage 80% is in C2.

The formula you want is =C2*A2-B2

 

[#3]

This worked for what I essentially needed it to do. Thank you very much! But one more question. I am using this formula to solve what is needed to get to that certain percentage for two percentages (let's just say 80 and 90%). I am finding a few instances where the answer from the formula is a negative number. Is there a formula I can use to show "N/A" if there is a negative number? Thinking this would be an If Then formula? Thanks!

[#4 · Accepted solution]

@Alindamood 

That would be

=IF(C2*A2-B2<0,"N/A",C2*A2-B2)

[#5]

That worked! Thank you!!