Multiple formulas for one cell

Trying to set up a cell to perform multiple formulas. Problem I'm running into is getting the cell to run both formulas or one or the other depending on the content of the cell.
Got the formulas down (I think).
Basically I want to have the number 20 assigned to cell C4 if cell C3 is greater than 100
Used this formula in C4:
=IF(C3<100,"20")
All good to this point.

The next part is ONLY if C3 is NOT greater than 100. So if C3 is 99.99 or below-I want to multiply C3 by D3
=C3*D3

Separately both formulas work fine. It's when I try to put them both in the same cell that things get screwy.
So I want a 20 assigned to C4 if C3 is 100 or greater, and stop running formulas if true. Only if C3 is less than 100 do I want it to run the multiplication formula.
What am I missing?

Your explanation is exactly what I'm looking for. Sorry I transposed the < > in the formula. I put the formula in exactly as you typed it...It doesn't seem to be recognizing the first part. My example--
Value of c3 is 102.04
so using the formula you provided c4 should be 20 with nothing further.
It's still multiplying c3 and d3(Value of d3 is 0.20) giving me 20.41 in c3