Teacher:In this task, you're given the title of a story consisting of five sentences, numbered 1 through 5. Your job is to determine which two sentences need to be swapped sentences in order to make a story that makes complete sense and is befittingly titled. Indicate your answer using the numbers of the two sentences in order, such as '34' or '25'. The first digit refers to the sentence which should come first in the story.
Teacher: Now, understand the problem? Solve this instance: Title: Lost at the Zoo. Sentence 1: When they got through the gate, they got separated. Sentence 2: Joe and his son went to the zoo together. Sentence 3: Joe looked everywhere, but could not find his son. Sentence 4: Eventually he went to the security office for help. Sentence 5: When Joe walked in, he saw his son sitting there crying.
Student:
21