mathschallenge.net

Problem

Although your mathematics teacher tells excellent jokes, he can be particularly cruel. At the end of a lesson he hands out a small piece of paper to each student.

A cement mixer collided with a prison van...

31 51 02 51 81 90 91 02 91 10
81 50 10 91 11 50 40 02 51 20
50 51 41 02 80 50 21 51 51 11
51 12 02 60 51 81 91 90 42 02
50 50 41 80 10 81 40 50 41 50
40 30 81 90 31 90 41 10 21 91

Can you discover the punchline?