## Solid Encryption

#### 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.`