The ratio of adult to student starts out being 140/210.
Both of these numbers are divisible by 10 so the first thing that happens is that you divide top and bottom by 10
140 ÷10 = 14
210 ÷10 = 21
14: 2 * 7
21:3 * 7
So both 14 and 21 are divisible by 7
14÷7 = 2
21÷7 = 3
The ratio is 2 / 3
Answer: For every 2 adults there are 3 students.