I have this question:
When performing a certain task under simulated weightlessness, the pulse rate of $42$ astronaut trainees increased on the average by $26.4$ beats per minute with a standard deviation of $4.28$ beats per minute. Construct a two sided $95\%$ confidence interval for the true average increase in the pulse rate of the astronaut trainees performing the given task.
This is what I worked out:
$ 26.4 \pm 2.021 \cdot \left(\frac{4.28 }{\sqrt{42}} \right) = (25.065, 27.735) $
I got this from taking the $95\%$ two sided confidence interval from the table on degree of freedom of $40$. I assumed this, because of my sample size being $42$, with the closes number being $40$.
I checked the answers I was given and they seem to use infinity for the degrees of freedom, my question is: when are we suppose to the infinity and when do we use the actual rows/degrees of freedom numbers?