A garden centre orders a summer lawn fertilizer once every year. Its customers do no prefer to buy fertilizers from the previous years stock. The cost of a fertilizer bag for the garden centre is $20. The centre sells the current years stock at $28 per bag and previous years stock at $18 per bag. The demand per year is 1,500 bags, which is normally distributed with a standard deviation of 110 bags. How many bags the centre should order each year