Replace the question mark with a number

For his final project in a statistics class, a student decided to conduct a survey. He chose to find out peoples' favorite pastimes.
The teacher required that he sample at least 100 people, so he started out his project visiting a fairly large apartment building near the university.
He knocked on the first door and a man answered.
"Sir, what is your name?" asked the student.
"John."
"Sir, I'm doing a school study and would like to know what is your favorite pastime?"
"Watching bubbles in the bath," came the reply.
He liked the esoterical answer and continued down the hall, until he came to the next door.
He asked again, "Sir, what is your name?"
"Jeff."
"Sir, Would you please tell me your favorite pastime?"
"Watching bubbles in a bath," was the answer.
Quite amused and confused, he went on to ask a good number of people in the building and all of them had the same pastime..."watching bubbles in a bath".
He left the building and walked across the street where there were several row houses, to continue the survey.
At the first house, he knocks and an attractive girl opens the door.
Our surveyor starts again - "What is your name?"
"Bubbles."