An astronaut visited mars. his weight on earth was 180 pounds, and his weight on mars was only 72 pounds. he removed a rock with a weight of 16 pounds on mars. what is the weight of the rock on earth?

The question given above may be answered through the concept of ratio and proportion. The ratio of the man's weight on Earth and Mars should be equal to the ratio of the rock's weight on Earth and Mars. By letting x be the rock's mas on Earth, the equation that would best represent the situation is,
                          180 / 72 = x / 16
The value of x is equal to 40. Thus, the rock weighs 40 pounds on Earth. 

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.