Data Sufficiency

Did it take Pei more than 2 hours to walk a distance of 10 miles along a certain trail? (1 mile= 1.6 kilometer, rounded to the nearest tenth.)

1) Pei walked this distance at an average rate of less than 6.4 kilometer per hour.
2) On average, it took Pei more than 9 minutes per kilometer to walk this distance.

Target question: Did it take Pei more than 2 hours to walk a distance of 10 miles?

This is a great candidate for rephrasing the target question.

Aside: We have a free video with tips on rephrasing the target question: https://www.gmatprepnow.com/module/gmat- ... cy?id=1100

If Pei were able to walk 10 miles in EXACTLY 2 hours, then Pei's average speed would EQUAL 5 miles per hour.
So, if it were to take Pei MORE THAN 2 hours, then Pei's average speed would be LESS THAN 5 miles per hour.

This means we can rephrase the target question as...
REPHRASED target question: Is Pei's average speed less than 5 miles per hour?

Since the two statements are in terms of kilometers, let's rephrase the target question one more time...

5 miles = 8 kilometers, so 5 miles per hour = 8 kilometers per hour.
RE-REPHRASED target question: Is Pei's average speed less than 8 KILOMETERS per hour?

At this point, it will be relatively easy to handle the two statements.

Statement 1: Pei walked this distance at an average rate of less than 6.4 kilometer per hour.
If Pei walked slower than 6.4 kilometer per hour, we can be CERTAIN that Pei's average speed less is than 8 kilometers per hour
Since we can answer the RE-REPHRASED target question with certainty, statement 1 is SUFFICIENT

Statement 2: On average, it took Pei more than 9 minutes per kilometer to walk this distance.
Let's see what it means for Pei to walk 1 kilometer in EXACTLY 9 minutes.
This means Pei walks 7 kilometers in 63 minutes, which means he/she walks a little SLOWER than 7 kilometers per HOUR.
Since we're told that it took Pei MORE than 9 minutes per kilometer, we can be CERTAIN that Pei's average speed less is than 8 kilometers per hour
Since we can answer the RE-REPHRASED target question with certainty, statement 2 is SUFFICIENT

Answer = D

Cheers,
Brent