Caclulation on linked field

Hi,

I’m doing some work for a children’s nursery have a field called “RATE PER HR” in the table STUDENT it is a currency and linked to a table called RATE. I need to do a simple calculation RATE PER HR * NUMBER OF HOURS I’m unable to do this without getting an error in the field. What am I doing wrong, how do I get around this ?

Thanks

Jason