Standard deviation question (Z or T test)

#1
Hi all!

Here's the question given to me:

ABC University claims that because of its superior facilities and close faculty supervision, its students complete the Ph.D. program earlier than is usual. They base this assumption on the fact that the national mean age for completion is 32.11. Their 31 Ph.D.s have a mean of 30.89 with a standard deviation of 4. Test the validity of their assertion.


Now here's my question, that standard deviation is unimportant, right? I need to do a T-Test right?
 
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