n = 36, the sample size
μ = 17, the population mean
σ = 6, the population standard deviation
When the random variable is x = 15, obtain
z = (x - μ)/(σ/√n) = (15 - 17)/1 = -2
From standard tables,
P(x <15) = 0.0228
Similarly, when x = 18, obtain
z = (18 - 17)/1 = 1
P(x <18) = 0.8413
Therefore
P(15 < x < 18) = 0.8413 - 0.0228 = 0.8185
Answer: 0.8185
