In a study of 250 adults, the mean heart rate was 70 beats per minute. assume the population of heart rates is known to be approximately normal with a standard deviation of 12 beats per minute. what is the 99% confidence interval for the mean beats per minute

This is a good question!

The steps are like follows:

Form the confidence level (99%), we calculate the "alpha" value
alpha=1-(99/100)=0.01

Now, calculate the critical probability (p-start)
p*=1-(alpha/2)=1-(0.01/2)=0.995

z-score could be calculated from p*,,,
z-score=2.57583
(http://www.socscistatistics.com/tests/ztest/zscorecalculator.aspx)

Now, calculate the Margin of Error:
standard deviation = s
number of adults = n

ME=(z*s)/(sqrt n)=(2.57583*12)/(sqrt 250) = 1.955

The confidence interval = mean +or- ME
                                      = 70+1.955  or  70-1.955

So that we can conclude that the 99% confidence interval for the mean beats per minute is 68.045 and 71.955

I hope you got the idea!

If you still need help, just let me know!