Gemma and Beth from Middlewich Primary School have sent in a very clear explanation of how they did this problem:
We put 5 in f's place then put 11 in g's place because if you
added 6 to 5 you get 11.
Then we took 11 from 13 to get 2 which is the letter a's
place.
We then worked out the letter l (4) by multiplying 2 by 2.
After doing this it was easy to work out the rest:
A-2
B-9
C-3
D-12
E-8
F-5
G-11
H-6
I-10
J-1
K-7
L-4