Can anybody solve the following A-level statistics question please? I can't see how to find the variance/standard deviation

Question

Data collected over a long period of time indicate that 23% of children contract a certain disease. Following a public awareness campaign, a doctor conducts a survey to find out whether this proportion has decreased. The doctor uses a sample of 3000 children and conducts a hypothesis test at the 2.5% significance level.

Use an appropriate normal distribution to find the approximate critical region for this test.

Answer 1

In order to compare a sample proportion to an population proportion, you assume a standard deviation of sqrt(p(1-p)/n) and apply a one-sided critical region to the left of -1.96 (2.5% to left of this critical value. From the definition of z* = (p*-p)/σ_p, you can solve for p* below which you can reject the null hypothesis and affirm the hypothesis that the proportion is less than p (97.5% confidence or at 2.5% significance)