Calculating t is simple, but I don't get how they calculate d.f.<br><br>The answer for d.f. is 9.96Suppose μ1 and μ2 are the mean stopping distances at 50 mph for cars of a certain type equipped with two different types of braking systems. Use the two-sample ttest at a 1% level of significance to test H0: μ1- μ2 = -10 versus H1: μ1- μ2 < -10 for he following data:

n1=6, x1=115.7, s1=5.03, n2=6, x2=129.3 and s2=5.38

EDIT: Problem solved.