No, you should not use the same approach from part (a) to calculate the expected number of smokers among these 27 students because these twenty seven students are not a random sample from the university’s total population, example it might be argued that the proportion of smokers among the 27 students outside the gym at 9 am on Saturday morning would be lower than the proper population of the university as a whole.
To add, a subset of a statistical population in wherein each member of the subset has an equal probability of being chosen is called a simple random sample.