For which situations would it be appropriate to calculate a probability about the difference in sample means?

1) Both population shapes are unknown. n1 = 50 and n2 = 100.
2) Population 1 is skewed right and population 2 is approximately Normal. n1 = 50 and n2 = 10.
3) Both populations are skewed right. n1 = 5 and n2 = 10.
4) Population 1 is skewed right and population 2 is approximately Normal. n1 = 10 and n2 = 50.
5) Both populations have unknown shapes. n1 = 50 and n2 = 100.
6) Both populations are skewed left. n1 = 5 and n2 = 40.