A combination is the weight of the two dogs added. If one dog weighs 15 kilos, while the other weighs 13 kilos, we must add 15 to 13. 15 plus 13 is 28, so the combination of the two dogs is 28.
Sorry for the drawing but what you do is add the boxes dont multiply so it will be t plus 5 plus 5 equal your answer or 3 plus 3 plus 3 plus 3 plus 3 equals your answer