Been doing probability all day and am a lil confused. I just made the following problem up. How would you go about the following problem? There isn't an OA and hoping someone can solve this problem using both probability and permutation/combination methods with steps explained. Thanks

In a class of 30 students, each of the students were asked to pick an integer between 1 and 20, both inclusive. What is the probability that at least two students have picked up the same integer?