It is given that the sample size is 17.
So degrees of freedom is 16.
The confidence level is 80%.
Use the t table or a tool to find the critical t value as below:
The one tail t value for 80 percent confidence is 0.865 using t table.
So the critical value is 0.865.
The answer is 38, because if you take away the 15 he bought afterwards (34-15) you get 19, and then since he sold half his collection you would double that (19×2) and that's 38.
