Approximately 75% of cars driven by company employees are older than what age? ...

Suppose ages of cars driven by company employees are normally distributed with a mean of 8 years and a standard deviation of 3.2 years. Approximately 75% of cars driven by company employees are older than what age?
about 10.2 years
about 5.9 years
about 4.8 years
about 2.1 years

Let X be the ages of cars driven by company employees.

X ∼ n(8; 3.2)

P(X ≥ x) = 0.75

P((X - 8)/3.2 ≥ (x - 8)/3.2) = 0.75

P(Z ≥ - z) = 0.75

z = - 0.675

- 0.675 = (x - 8)/3.2

x = 5.84years!