So here is how we are going to get the answer. Given that the astronauts weight is 180 pounds on earth and 72 pounds in mars, we can put it like this: 180:72. If a rock is 16 pounds on mars, we are going to find the weight of the rock on earth by cross multiplication. So, 180 : 72 is to ? : 16. So the answer would be 40. The answer therefore would be 40 pounds. Hope this helps.