Picklist in a formula field

Picklist in a formula field

Hi 

Can Picklist be used in a Formula field?

There is a bug in the Forumla field:

1. I've a currency field called "Total Fee". 
2. There is a picklist field called "Installments Opted" with values { 1 , 2 ,3 ,4 }
3. I want to create a formula field called "Ist Amt Due" for which the logic is:

If Inst Opted = 1 , then Ist Amt Due = Total Fee * 100%
Else If Inst Opted = 2 , then Ist Amt Due = Total Fee * 60%
Else If Inst Opted = 3 , then Ist Amt Due = Total Fee * 50%
Else If Inst Opted = 4 , then Ist Amt Due = Total Fee * 40%

From my point of view, this is a simple and straight forward logic but I keep getting error when creating the formula field. It either says "syntax error" or "invalid return type". 

Can you please check if there is a bug in the product or If i'm getting something wrong? 

Thanks