I need help figuring out this problem. Please help me by answering and showing me explanations.

Problem 1. You have a collection of three surveys, and you need to recruit 20 participants for each of them. To recruit participants you stand on the West Mall and ask people if they want to take your survey. For the rst survey, you have to ask 35 people before you get 20 participants; for the second, you have to ask 45 people; and for the third you have to ask 50 people. Assuming that each person's decision to take the survey or not is independent of any other person's decision, what is the maximum likelihood estimate of the probability that a randomly selected person will agree to participate in your surveys