Q:

Formula for data that looks like a hockey stick

The following are X and Y values:
X    Y
20   10
100  25
140  50

It would be nice if I could approximately represent this data as a formula and plug in any arbitrary value of X from 20 to 140.  The data seems to be kind of like a hockey stick curve.  Or is it more exponential?  I could code this to just create linear segments between those three points but it would be way better if a formula could give me more of a smooth result.  The numbers above aren't fixed in stone.  They can vary a little.  As long as the formula gets me in the ballpark. 

A:

You can use the equation of a polynomial or an exponential to approximate that kind of shape, which one you pick sort of depends on what exactly you are trying to do. 
If you just need a smooth curve that is close to some shape, then pick a formula and tweak the coefficients until it looks how you want. 
A polynomial fit of your data from Excel gives:
Y = 0.0036*(X^2) - 0.25*X + 13.542

