Excel Solver Issue

[Zacross]

Hello to everyone,

I'm a first-yeat student in Business&Economics. My "homework" for this week-end is to solve problems that can be linearly written using the Add-in Solver of Excel
However, although I am pretty sure my model is fine, Solver detects an error in the objective or contraints cells.
It is quite hard to explain how I build the model without showing the problem...
The only thing I can say is that when I did all the constraints, I have a "Div/0" issue and I have no idea how to fix it...

What can I do? Thanks in advance

edit : As I see, I can attach a file =) It will give you a better overview of what I did exactly, and maybe allow you to help me out...
Is my model wrong? Or is there a subtility I don't know about to make that work?
I'll summarize quickly the actual problem in case the model I made make no sense to you without its context

I must "build" a special chemical product with 3 Compound at the lowest cost.
Compound 1 : 5$ - 20% sulfur - 60% Iron - 20% Potassium
Compound 2: 5.25$ - 40% sulfur - 30% Iron - 30% Potassium
Compound3 : 5.5$ - 10% sulfur - 40% iron - 50% Potassium

The final products has to contrain at least 20% sulfur, 30% iron and 30% potassium, but having less than 45% potassium