Find the range of values that represent the middle 99.7% of distribution

The middle 99.7 % of data in a normal distribution is the range from µ - 3σ to µ + 3σ. The standard deviation is 55, so 3σ = 3 ∙ 55 or 165. Therefore, the range of values in the middle 99.7 % is 251 < X < 581.