Run a slice of grapefruit around the rim of a short tumbler. Pour a bit of salt onto a small plate. Dip the rim in salt. Fill the tumbler with ice.
Add tequila, triple sec, agave, lime juice, and grapefruit juice. Stir well until the drink is very cold. Place a small grapefruit slice on the rim for serving.
Recipe by Tastemaker Blog at https://tastemakerblog.com/2018/03/08/grapefruit-margarita/