A school uses a phone tree to communicate updates to families.​ First, the 3 school administrators decide when an announcement is needed. They each call 5 people in the first round of calls.​ Then, each of those people calls 5 people in the second round. In every​ round, each person who received a call in the previous round makes calls to 5 new people. Let x be the number of​ rounds, and let y be the number of people called during the round. Write and graph a function that models this situation. Label the​ y-intercept, and explain what it represents.