A group of students is on sightseeing tour. The total fare is $129 and this is to be shared equally among the students. If two more students join the tour, each will pay $2 less. Find the original number of students in the group.

Let be x the number of students: the original price $129 is to be equally divided, thus each student will pay $129/x. If there were 2 more students, thus x+2 participants, the per capita cost will be $2 less, thus $129/x-2. Thus you have
(\frac{129}{x}-2)(x+2)=129
and solve in x.