Unclear what you want. Do you want to calculate the number which, when mult. by "1/2 gallon," produces "2 3/4 gallons?"
If so: Call that number x. Then
2 3/4 gal 11/4 gal
--------------- = x = ------------- = 11/2
1/2 gal 2/4 gal
in which case your "2 3/4 gallons =_________1/2 gallon" becomes
2 3/4 gallons = (11/2)*(1/2 gallon), read as "5 and one half half gallons."