This is a common example of representative scale size, or sample size. In selecting a small amount of people at random from a larger group, one can estimate the amount of people in the large group picking a specific answer based on a smaller, but proportionate, group size.
In the sample size, 21 of the 45 tested people said that they would go on more than three vacations a year, and this scale is supposedly proportional to the total group. Therefore, we can use a simple proportion to estimate the amount of people who would also say that they are going on more than three trips a year:
21 would ‘N’ would ————— = —————— 45 total 516 total
where N represents the number of people in the big group that would go on more than three trips a year.
Solve the proportion by cross-multiplication:
45N = 10,836 N = 240.8 people, or 241 people.
Based on the sample data, 241 people out of the 516 total will be taking more than three vacations a year.