.
The skier covered the distance of 7 kilometers between the 3-km mark and 10-km mark in 40 minutes.
40 minutes = 
of an hour.
Therefore, the average rate of the skier is the ratio
average rate = 
= 
= 
= 
= 11.5 miles per hour.
Answered, solved, completed and explained.
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The note of significant importance :
The combination of words "average rate of change" is not a synonym to "average speed" or "average rate".
This combination of words "average rate of change" is never the term of usage, when you talk about
"average speed" or "average rate of movement".
The only right and correct terms in this case are "average rate" or "average speed".
Watch your language !
If you use incorrect terms, you demonstrate that you don't know a standard terminology.
