What is the test statistic zstat= ?

Question

A cellphone provider has a business objective of wanting to determine the proportion of subscribers who would upgrade to a new cellphone with improved features if it were made available at a substantially reduced cost. Data are collected from a random sample of 400 subscribers. The results indicate that 99 of the subscribers would upgrade to a new cellphone at a reduced cost. Reducing the price will be profitable if at least 20% of the subscribers would upgrade

1. at the 0.05 level of significance is there evidence that more than 20% of the customers would upgrade to a new cellphone at a reduced cost?

the answer is:

h0 pi < or equal to 0.20

h1 pi > = 0.

2). What is the test statistic here? Zstat = ?

Answer 1

z = (0.2475 - 0.2)/sqrt(0.2 * 0.8/400)