Answer: 135 days
Explanation:
Since the amount of time it takes her to arrive is normally distributed, then according to the central limit theorem,
z = (x - µ)/σ
Where
x = sample mean
µ = population mean
σ = standard deviation
From the information given,
µ = 21 minutes
σ = 3.5 minutes
the probability that her commute would be between 19 and 26 minutes is expressed as
P(19 ≤ x ≤ 26)
For (19 ≤ x),
z = (19 - 21)/3.5 = - 0.57
Looking at the normal distribution table, the probability corresponding to the z score is 0.28
For (x ≤ 26),
z = (26 - 21)/3.5 = 1.43
Looking at the normal distribution table, the probability corresponding to the z score is 0.92
Therefore,
P(19 ≤ x ≤ 26) = 0.92 - 28 = 0.64
The number of times that her commute would be between 19 and 26 minutes is
0.64 × 211 = 135 days