A grocery shop owner claims that the mean of his daily sales is not more than OMR 80, but a tax agent believes the owner is untruthful. Observing a sample of 20 days, the agent finds the mean purchase to be OMR 91, with a standard deviation of OMR 21. Assuming the population is approximately normally distributed, and using the 0.05 level of significance, is the agents suspicion confirmed?