There is a video on khanacademy, search "rationalize the denominator." Then instead of getting the answer you will learn how to do it and be able to do it on other problems in the future.
Answer:
(30.09, 32.91)
Step-by-step explanation:
Mean number of pounds of trash = x = 31.5 pounds
Standard deviation = s = 7.8 pounds
Sample size = n = 120
Confidence level = 95%
Since, we are given the sample standard deviation instead of the population standard deviation, we will use t-test to find the confidence interval.
Degrees of freedom = df = n - 1 = 120 - 1 = 119
The t-value associated with 95% confidence interval and df=119 from the t-table comes out to be:
t = 1.98
The formula to calculate the confidence interval is:

Using the values in above expression we get:

Thus, the 95% confidence interval for the mean number of lbs of trash per person per week that is generated in the city is (30.09, 32.91)
I think it's C. drawing a king from a deck