another approach can be: put the corresponding numbers in place of all the alphabets, then do additions and subtractions with only the adjacent corner values to get the middle number.
Like,
20-5=15 (subtracting 1 from it we get 14)
8+5=13 (adding 1 to it, we get 14)
24-8=16 (subtracting 2 from it we get 14)
24-9=15 (subtracting 1 from it we get 14).
So, d (value: 4) should be placed such that 20-4=16 (subtracting 2 from it we can get 14) and 9+4=13 (adding 1 to it, we can get 14).