I'm stucked at the following challenge #6 because this one's implementation is wrong, as anything > 0.1999 will pass.

Can someone look at my implementation and tell me where I'm failing?

from math import sqrt
from scipy.stats import norm

init_supply = 74000
mu = 50000.
sigma = 10000
weeks = 11
delivery = 47000
sample_mu = 20000.

mu_11w = init_supply + (delivery-mu)*weeks
sigma_11w = sigma * weeks

diff_mu = sample_mu - mu_11w
z = diff_mu / sigma_11w
print(format(norm.cdf(z), '.4f'))

Below the calculations to support the choice of my equations (init_supply → supply, delivery → X, weeks → w)

mu_11w = supply + (X-mu)*w

sigma_11w = (supply + (X-mu+sigma)*w) - (supply + (X-mu)*w)
          = supply + X*w - mu*w + sigma*w - supply - X*w + mu*w
          = sigma*w

diff_mu =  sample_mu - mu_11w

z = diff_mu * sqrt(N) / sigma_11w
N=1 ⇒ z = diff_mu / sigma_11w