A fish store buys fresh tuna daily for $4.20 per kg and sells it for $5.70 per kg. At the end of each day, any remaining tuna is sold to a producer of cat food for $2.40 per kg. Daily demand for tuna at the fish store can be approximated by a Normal distribution with a mean of 80 kg and a standard deviation of 10 kg. What is tire optimal stocking level?