The 60th visiter 
Here’s why—
First, list the multiples of 6 and 20 to find the least common multiple. 
6– 6, 12, 18,24,30,36,42,48,54,60,66,72 
20– 20,40,60,80,100 
The Least common multiple is 60, telling us that the 60th visiter will get a free calendar and book at the same time.