The expected value of the cookie pack is 250 g. From the 1000 packs sampled, the average weight was actually 255 g with a standard deviation of 2.5 g. That means, the z-score for this distribution is
z = (250 - 255)/2.5 = -2
Now, we're looking for the percentage or P-value for the packs that weigh less than 250 grams. Using the P-value table, we have
P(z < -2) = 1- P(z < z ) = 1- 0.9772 = 0.0228
Thus, the 2% of the sample weighs less than 250 grams.
