The formula for z-score is:
z-score = (X – μ) / σ
where,
X = sample
μ = mean = 1001
σ = standard deviation = 6
Since we are looking for volumes less than 1001 mL, let us use 1000.5 mL for X.
z-score = (1000.5 – 1001) / 6
z-score = -0.083
Referring to the standard distribution table for z-test, this corresponds to:
p = 0.4681
So 46.81% of the bottles are less than 1001 mL.