The interval of times that represents the middle 99.7% of his finishing times in the 400 meter race is; (73.5, 87.5)
We are given;
Mean; E(X) = μ = 80 seconds
Standard Deviation; σ = 2.5
From the empirical rule, the probability of obtaining values within one deviation from the mean is 0.68; within two deviations is 0.95 and within 3 deviations from the mean is 0.997.
We have 99.7 % of the values within 3 deviations from the mean in our question and so the intervals are;
μ - 3σ = 80 - 3(2.5) = 73.5
μ + 3σ = 80 + 3(2.5) = 87.5
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