well, 3900 is 1.107σ below the mean.
Now just head to your Z table to interpret that as a probability, then multiply that by 365
A petrol station finds that its daily sales, in litres, are normally distributed with mean 4520 and standard deviation 560.
(a) Find on how many days of the year (365 days) the daily sales can be expected to exceed 3900 litres.
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