What are the degrees of freedom for an independent-samples t-test using one sample with n = 13 and ...

What are the degrees of freedom for an independent-samples t-test using one sample with n = 13 and one sample with n = 15?
 
  What will be an ideal response?

Question 2

What are the degrees of freedom for an independent-samples t-test using two samples with n = 20 in each sample?
 
  What will be an ideal response?

Question 3

When is true, what does the mean of the sampling distribution of differences between means equal?
 
  What will be an ideal response?

Question 4

Suppose you tested two age groups on the number of details they could recall from a paragraph. The mean for the older group is 16, and the mean for the younger group is 14 . Further suppose you fail to reject the null hypothesis for this independent-samples t-test. What best accounts for the difference between these sample means?
 
  What will be an ideal response?