23/4 gallons =/2 gallon

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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."