Question 971479
mean=13.8 d   SD=4.3 d

z< (16-13.8)/4.3 =(2.2/4.3)

z<0.51
p=0.695

95% from the left is z=1.645
z* sd= 1.645 *4.3=7.07 days

13.8+7.07=20.9 days

z>(16-13.8)/(4.3/sqrt(29))
z>2.2/.798  =2.755

p=0.003

One invoice can certainly be more than 16 days but the MEAN of 16 of them is far less likely.