so the formula we were given was Amortization=(Lr)/(1-(1+r)^(-t)) where t is time r is rate and L is lump sum.... the problem goes like this... i could either buy a toy for 500$ or pay 200 $ each month for 3 months.... so i already have the t value=3, L which=500$ and A which=200$ HOW DO YOU SOLVE FOR THE INTEREST RATE? I'VE TRIED TO DERIVE THE FORMULA but i just cannot.. I think we have to use the ratio and proportion technique... HELP!!