Answer: (13.4996, 32.5004)
Step-by-step explanation:
The confidence interval for mean difference is given by :-
,
where
= sample mean difference .
= critical t value (two-tailed)
SE= Standard error.
Given :

SE= 4.2
Now, the confidence interval for mean difference will be :-
Hence, the required confidence interval : (13.4996, 32.5004)
Im pretty sure the answer is c) 136
He will bye 16 pens hope that helps u
Its -3 or 3 sorry if you get it wrong if its not tight.