I got the following question given below. According to me this should be solved using z test as the sample size is more than 30. But they are using t test. What is the correct method z test or t test? Please advice

A social worker wants to know whether the mean annual salary of her clients

matches the mean annual salary for all city residents, which is $28,000, or whether

it is less. She obtains a random sample of her clients salaries of size 36. The

mean of the sample is $27,500 and the standard deviation is $1,200. Perform a

hypothesis test at the 5% level of significance

Thanks

Simi