Zoho Creator Formula variable lookup based on selection in a Drop Down list

Zoho Creator Formula variable lookup based on selection in a Drop Down list

I have a Zoho Creator form for Storage Tank Inventory Tracking where the first field is a drop down box with selections like "Tank1 / Tank2 / Tank3 etc." 

Another field is where the user enters the inventory level in Gallons. 

I have a formula field for automatically computing the lbs of inventory (Gallons * Density) but the density varies depending on what Tank the user has selected in the Drop Down. 

If Tank1 then Density = 1.2. If Tank2 then Density = 1.5 etc.

How do I code this most elegantly?