$$\begin {array}{c c c c} 20&6&7&17\\9&15&14&12\\13&11&10&16\\8&18&19&5 \end {array}$$
Looking at above matrix you can find the values of A to L.
Construction of a 4x4 magic square can be found in a book by Jerome S. Meyer, Fun With Mathematics.
Method (slighly modified for your input) goes as:
- Count 20 to 5 and fill only the digonal numbers
$$\begin {array}{c c c c} 20&-&-&17\\-&15&14&-\\-&11&10&-\\8&-&-&5 \end {array}$$
Not visited numbers: 19, 18, 16, 13, 12, 9, 7 and 6
Now go in reverse direction counting non-visited numbers and fill in the empty cells to get the answer matrix.
This works because you are balancing the sum of pair of numbers available before with the pair of new numbers in opposite order. (Equal distribution)