To qualify, what must a woman's average speed have been in meters per second? there are 1609 m in a mile.

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.