Question 1163255
with s use a t-distribution
t (df=18)=-3/6.4/sqrt(19)=-2.04
p=0.056

with n=24, the t=-2.30 and 
p=0.0311

with the third 
t=-3/6.9/sqrt(35)=-2.57
p-value is 0.0146

The fourth is the same, since the t-distribution is symmetric  and the difference from the mean is +3 now, rather than -2.