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Quantity A is 20-10=10cm and Quantity B is 9-4=5sec. the average rate of its position with respect to the clock time is 2cm/sec.

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You definitely have the right idea, and your final result is correct. However your terminology does not quite fit the definition of average rate, and it's avoids a lot of confusion in upcoming assignments to get the terminology correct from the very beginning.

The average rate of change of quantity A with respect to quantity B is (change in A) / (change in B).

Using the terminology of your solution your calculation would have been Quantity A / Quantity B.

The problem is that what you identified as Quantity A was in fact the change in quantity A; similarly for Quantity B.

As a result you got the right answer, but with not-quite-correct terminology.

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The following statement, modified from yours with just a added few words, is correct:

The change in quantity A is 20-10=10cm and the change in quantity B is 9-4=5sec. the average rate of its position with respect to the clock time is 2cm/sec.

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In any case, quantities A and B would be identified by their names. Quantity A is position, and quantity B is clock time.

With this terminology the definition is a perfect fit.

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