I did it using variables
I think more than one of us probably breathed a sigh of relief when we read that. I can understand why your instructor might not have wanted to introduce them yet but to ban them seemed a little crazy.

instead of 12 I divided by 1200 and it worked
Ah, so you were pulling the value for the monthly interest rate wrong. By the way, that probably means you could just take the formula as you had it in post 16 and change the 12 to a 1200 and get the right answer. That would mean you wouldn't be using variables. I personally think it would be a backward step but it might keep your instructor happy.