The fact that the mean is already < 40% doesn't make it impossible to reject the H0?
In this exercise we would like to know if the population mean of the competitor is higher than 40%. But the fact that the sample mean is already under 40%, doesn't it mean that it's impossible to conclude that the global mean is > 40%?
For example, there is no situation where the confidence interval would fall entirely > 40%, since the sample mean is under it.
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