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