Mr. R.’s World of Math

THEN CAME SATURDAY MORNING…

The students arrived at school early on Saturday morning and they were all in a bad mood. Well, they were all in a bad mood except for David who would have liked to have had school eight days a week.
In the classroom, their teacher, Mr. R. sat between his purple swirls and pink polka-dots as he waited for the students. He was not in a good mood either. He was upset the students had painted his room purple, and he was upset he had to be in school on a Saturday just as much as the students were…
Maybe it was because the students were used to sleeping late on Saturdays, or because they were just tired, but when Mr. R. started teaching, a few of the fourth graders had some problems staying awake.

It was already 1:15 in the afternoon when Justin finally woke up.
     “Where am I? What time is it?” asked a confused Justin.
     “You happen to be in school and it’s 1:15” whispered Serena, “you’ve been sleeping since math started at 12:30. Now we’re starting science.”

How many minutes had Justin been sleeping?