Add the Roman numerals:
XXXVIII + XII =
L
Combining all the letters of the two roman numerals, XXXVIII and XII, we get:
XXXX V IIIII
IIIII can be written as V, So the number becomes
XXXX V V
Now, the VV can be written as X, So the number becomes XXXXX.
Now the XXXXX can be written as L.
Hence, XXXVIII + XII = L.