Question 1151691
Assume randomness and sample's being normally distributed

Ho: Mean is >=4.0
Ha: Mean is < 4.0
test stat is a t df=99 0.95
critical value is t <-1.66
t=(3.8-4)/0.46/sqrt(100)
t=-0.2*10/0.46=-4.34
This is highly significant, and the buyer's complaint is supported at the 5% level