Baumol model followup

So sir are the below 2 formulae correct and can it be applied in all Baumol model questions? 1) Interest lost on investment which we calculated by Interest rate on investment (%) multiply by (Investment sold at start of year + cumulative investment sold at end of year) / 2 2) Interest earned on bank balance which is calculated by (interest earned on bank balance (%) multiply by investment sold each time) /2