Variables based on other variables

I'm trying to set up an activity where the user enters the weight of an item (this is variable UW), then enters the amount of items in a particular location (a bin). I'm trying to make it so the variable BWT is always equal to (BW * UW).  However, the system keeps calculating it as (BW * BW). I've attached the source files. Can anyone tell me what I'm doing wrong? Thanks.