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?

Solution

The method of encryption is to convert each letter to a number according to its alphabetical position, then reverse the digits:

A = 01 = 10
B = 02 = 20
C = 03 = 30
D = 04 = 40
E = 05 = 50
F = 06 = 60
G = 07 = 70
H = 08 = 80
I = 09 = 90
J = 10 = 01
K = 11 = 11
L = 12 = 21
M = 13 = 31
N = 14 = 41
O = 15 = 51
P = 16 = 61
Q = 17 = 71
R = 18 = 81
S = 19 = 91
T = 20 = 02
U = 21 = 12
V = 22 = 22
W = 23 = 32
X = 24 = 42
Y = 25 = 52
Z = 26 = 62

Using this method in reverse to decode the punchline:

A cement mixer collided with a prison van...

Motorists are asked to be on the lookout for sixteen hardened criminals.