Answer:
980 miles
Explanation:
If Alejandra averaged 70 mph for the trip, she drove 70 miles for each of the 14 hours, or ...
(14 h)(70 mi/h) = 980 mi
Her total distance was 980 miles.
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Comment on speed limit vs average speed
You are correct in your assessment that you need to know how fast Alejandra drove. Since that is not given in the problem statement, assumptions are required. I expect the assumption is supposed to be that her speed was at the speed limit.
In my experience, the average speed for a trip is 10-20 mph lower than the speed limit, due to stops for gas and eating. This problem statement seems to only count driving hours, so perhaps her average speed is very near the speed limit.
Though illegal, it is also not unusual for drivers these days to drive 5-10 mph over the speed limit. The "flow of traffic" is generally recognized to be 7 mph over the speed limit for speed limits above 30 mph.