Actually the teacher didn't even know you could use either addition or subtraction in comparing problems (where the difference is unknown, that is). "Joining" problems are addition; "separating" problems are subtraction; and "comparing" problems you can use either (like I said before, these are not my rules, just what the teaching text says). She told him "you can only use subtraction in comparing problems." I think it is actually a matter of her not really knowing the material she is suppose to be teaching. [As the Annenberg text says "Compare problems involve a comparison of two distinct, unconnected sets. Like the part-part-whole problems, compare problems do not involve action. A relationship of difference, more than, or less than is found in compare problems. Where the difference is unknown, either subtraction sentence or an addition sentence is correct. Example problem: Ahmed has two brothers. Christine has three brothers. Christine has how many more brothers than Ahmed? Example Answers:
3-2=1 or 2+1=3"]

Last edited by Irena; 09/26/13 04:27 PM.