what is the field type for "Drop down Box

what is the field type for "Drop down Box

Hi...Guys..

By using "Drop down Box" field and the values in the field are "Numbers" , then Is it possible to perform  "Calculation - Task" 

or 

If I put it another way , what is the field type for "Drop down Box" ??

Here, Scenario, 

Live Form Window is Guest Book - Room Booking.

So, when user select the Guest Name ---> Booking type ----->> Rooms Available ---->> No.of Persons. 

Base on the --->> No. of Persons,  Rate should be shown 

Then Room Charges , Calculation Task has to be Perform.

Instead of "Drop down Box" field, I am getting an "Error". I have attached the Image below. 

If I keep " Number " field, then Calculation part is working fine.

I want to know the reason... why it's not for "Drop down Box" ??

Kindly Let me know

Thanks & Regards

Prashanth K