Given:
μ = 89 ng/ml, population mean
σ = 23 ng/ml, population standard deviation
Random variable, x = 100
To test P(x < 100), calculate the z-score.
z = (x-μ)/σ = (100 - 89)/23 = 0.4783
From standard tables, obtain
P(x < 100) = 0.6838 = 68.4%
Answer: 68.4%