The amount of regular unleaded gas purchased every week at a particular gas station follows the normal distribution with a mean of 50000 gallons and a standard deviation of 10000 gallons. the starting supply of gasoline is 74000 gallons, and there is a scheduled weekly delivery of 47000 gallons. how many gallons should the weekly delivery be so that after 11 weeks the probability that the supply is below 20000 gallons is only 0.5%?