Thanks for your quick answer.
If you mean the option "Question Level Simplify" (towards the bottom of the page where I edit the question), turning this on or off doesn't make any difference.
Kind regards, Ines
I meant the last entry for "Input ans1". If I add the option "simp" there, it causes exactly the behaviour you describe. Otherwise the input "a+b+c" is also interpreted as "a+b+c".
So I guess that you have some kind of systemwide input simplification. Maybe others know more ...