1 cup boiling water
1 (3 ounce) package strawberry-flavored gelatin (such as Jell-O®)
1 (.25 ounce) package unflavored gelatin (such as Knox ®)
2/3 cup cold water
1/3 cup mango-flavored rum
Mix boiling water, strawberry-flavored gelatin, and unflavored gelatin together in a bowl until dissolved. Mix in cold water and mango-flavored rum. Pour into 1-ounce plastic or paper cups.
Refrigerate gelatin mixture until set, about 1 hour.