How do we find an interest rate?​

Assume that (i) setups need to be completed first; (ii) a setup can only start once the batch has arrived at the resource, and (iii) all flow units of a batch need to be processed at a resource before any of the units of the batch can be moved to the next resource. process step 1 molding 2 painting 3 dressing setup time 15 min. 30 min. no setup processing time 0.25 min./unit 0.15 min./unit 0.30 min./unit which batch size would minimize inventory without decreasing the process capacity?