What statistical test and how many of them will need to be used to explore the difference in average cholesterol within each group before and after the treatment (i. e., how much did cholesterol level change within each group as result of the treatment) ?

The statistical test to be used is the paired t-test.

Step-by-step explanation:

The dependent t-test (also known as the paired t-test or paired-samples t-test) compares the two means associated groups to conclude if there is a statistically significant difference between these two means.

We use the paired t-test if we have two measurements on the same item, person or thing. We should also use this test if we have two items that are being measured with a unique condition.

For instance, an experimenter tests the effect of a medicine on a group of patients before and after giving the doses.

Similarly, in this case a paired t-test would be used to deter whether there was any changes in the cholesterol level within each group as result of the treatment.

Thus, the statistical test to be used is the paired t-test.