Most of my seventh grade students solved this fairly quickly. It was interesting to see a very common wrong answer of 6,358,563. Can you guess what they did wrong?
We did discuss whether or not all of the elephants would die. This is probably more intriguing as a science question than as a math question. How many would die from a lack of oxygen? How many would die from being crushed by other elephants? Would they explode from the lack of air pressure? How many would float for a bit, slowly be brought down by Earth's gravity, and burn up in the atmosphere? I'm not sure I know the answers to these questions, but it was interesting to see the kids hypothesizing.
By the way, one of my students thought that there would be eleven elephants. She said she just counted them on the picture. I told her that if she sees an elephant that big, she better hope that it doesn't have a bowel movement on top of her. (There's my sequel. How big would that elephant's bowel movement be?)