A customer has been ordering $5000$ unit at the rate of $1000$ unit per order during last year. The production cost is Rs$.12$ per unit $--$ Rs$.8$ for material and

labor and Rs$.4$ overhead costs. It costs Rs$.1500$ to set up for one run of $1000$ units and inventory carrying may buy at least $5000$ units this year, the company would like to avoid making five different production runs. Determine the most economic production run.