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.
Let p be the true proportion of US adults who believe raising the minimum wage will help the economy. A point estimate for p is and a good aproximation (because we have a large sample) for the standard deviation of is . Therefore, a 99% confidence interval for p is given by where and is the th quantile of the standard normal distribution, so we have, , i.e., or equivalently (0.3798, 0.4602).