formula in one cell or another
 

When the user enters a number in cell A1, I want cell B1 to divide that
number by 8. If A1 is blank then B1 should be blank.

Alternately, when the user enters a number in cell B1, I want cell A1 to
MULTIPLY that number by 8. If B1 is blank then A1 should be blank.

I'm pretty sure this has to be a workbook function but I'm not sure how to
proceed. Any ideas all?