Answer:
(A) 1100 and 1300 hours
Step-by-step explanation:
In a normal distribution, we can say that 68% of the values is between the range [µ-σ;µ+σ] with µ = the mean and σ is the standard deviation.
95% of the values are between the range [µ -2σ; µ+2σ] = [1100;1300]
99.7% of the values are between the range [µ -3σ; µ+3σ] = [1050;1350]
To find 75% of the values, we have to use the z-score
for 75% the Z-score = 1.15
This gives the range: [µ -Zσ; µ+Zσ] ⇒ [1200 - 1.15*50;1200+1.15*50] = [1142.5 ; 1257.5]
We can say that at least 75% (or more) is in the range [1100;1300]
