If function and capability

I'm working in Excel right now trying to create a spreadsheet for window pricing. 

Quick example, if I'm building a quote for a window install and the option from the drop down menu is single opening, then the rate is populated in one column. But that rate needs to be multiplied by the quantity and equal a total amount.

My activity (type of window) is in column B, quantity of that type of window is in column E, the rate for that type of window is in column F, the total for that type of window should display in column G. I know I could simply put G16=F16*E16, but what if the type of window is blank? How do I make column G blank and not multiply anything?