According to the 68, 95, 99.7 rule, 99.7% of the data is in the range:
(μ - 3σ, μ + 3σ)
where μ is the mean, and σ is the standard deviation
In this case, μ = 27 and σ = 5. Then, this range is:
(27 - 3x5, 27 + 3x5)
(12 , 42)
99.7 % of the customer accounts have payment made between 12 and 42 days.
