OK, you start off with the word SIX then subtract and/or add Roman numerals, therefore SIX minus IX(9) equals S, subtract X(10) from the IX(9) leaves I, add (40)XL, subtract (50) L leaves X(not 10). Answer is S I X. I found this in a book that was printed in the 1890's and have been trying to find it eversince. Your answer by using a calc. is correct. Trick question if you like. Also when you add (40) XL it is NOT looking for an answer there, techincally at that point it would be (41)XLI, subtracting (50) L would leave XI(11)
I'll let you lick your thumb and smooth your brow on this one. Hopefully this has clarified it albeit confusingly, look at the logic.