Question 1120471
The mean is 9
Assuming the sd is known is consistent with using a z-test
the 90% interval will be mean+/-1.645, the z value for 90%*2.1/sqrt(15).  The 99% interval will be using 2.576 in place of 1.645
The intervals are (8.1, 9.9) for a 90% CI.  To be more confident, one needs a wider interval and the 99%CI is (7.6, 10.4).  Units are minutes.