what is the z value ?

Question

Based on historical data, your manager believes that 30% of the company's orders come from first-time customers. A random sample of 115 orders will be used to estimate the proportion of first-time customers. What is the probability that the sample proportion is less than 0.28?

Note: You should carefully round any z-values you calculate to 4 decimal places to match wamap's approach and calculations.

Answer 1

z = (phat - p)/sqrt(p * (1-p)/n)

here phat = 0.28, p = 0.3, n = 115.

P(phat < 0.28) = P(z < z computed above)