# Blog

## Coded message brainteaser

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You’ve just received a secret coded message:

AD, BA, BH, CE, DB, DI, EF

You know it follows a pattern, but can you work out what pair of letters come next?

You might have noticed that the first letter in each term is further along the alphabet. This could be considered a clue. And what if the pairs of letters represented numbers?

You can convert the letters to numbers using a simple substitution cypher:

A = 1, B = 2, C = 3 and so on.

This gives us the sequence:

14, 21, 28, 35, 42, 49, 56

You might notice that they’re all multiples of seven! That means the next number in the sequence is 63.

Taking it back through the substitution code, we get the answer, FC.

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## 5 responses

1. Matthew Ludowici

I did it differently but still got it, as the number of letters for the first one goes 1, 2,1,2,1 and goes up A,b,c etc. while the second letter in each increase by one letter. The letters that have one letter so the A,C and E have the second letter D,E,F so goes up one while the two numbers go up in the same order but start at A and H and therefore when you get to the next one in the order it is FC.

1. David

That’s a clever pattern! It might reveal something special about the seven times table. Have you tried extending the pattern even further? does it still make sense?

2. Abygael, 9

What a great code challenge for us to do over lunch!

3. Matthew Ludowici

I looked into the pattern further and it holds up.
BA 21
BH 28
CE 35
DB 42
DI 49
EF 56
FC 63
FJ 70
GG 77
HD 84
HK 91
IH 98
JE 105
JL 112
KI 119
LF 126
LM 133
MJ 140
NG 147
NN 154
OK 161
PH 168
PO 175
QL 182
RI 189
RP 196

I’m not sure as to why it works for all the letters but would be interesting to find out why it does that. Do you have any ideas?

4. Tanisha Ahmed

I love these brainteasers like wtf

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