Irena I don't think that Dude is arguing with you on the ways the question could be solved - but that the two parts of your sons answer do not match. Your son says "How many more legos are in the new set?" the answer to HIS sentence must be 10, your son is the one that formed a sentence to which the correct answer is subtraction not addition. Dude's problem is NOT directly with what the teacher asked, or the teacher's understanding, but with the internal consistency of your son's answer.