Please explain when a ztest would be appropriate over a ttes

Please explain when a z-test would be appropriate over a t-test.

Solution

First case:

When population standard deviation is known then z-test would be appropriate over a t-test.

Second case:

When population is noramlly distributed and sample size is large (greater than 30), then z-test can be used instead of t- test even if population standard deviation is unknown.