Prepare and bake cake mix according to package directions; cool completely. In a large bowl, dissolve gelatin and 3/4 cup sugar in boiling water. Mash half of the strawberries; add to gelatin mixture. Refrigerate until slightly thickened, about 1 hour. Slice remaining strawberries; stir into the gelatin., Cut cake into 1-in. cubes. Place half in a 3-qt. trifle or glass bowl. Top with half of the gelatin mixture. Repeat. Cover and refrigerate until set, about 4 hours. In a bowl, beat cream until soft peaks form. Gradually add remaining sugar, beating until stiff peaks form. Spoon over gelatin.