How many hours did it take Helen to drive from her house to

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GMAT Prep

How many hours did it take Helen to drive from her house to her parent's house?

1) Helen's average speed on this trip was 72 kilometers per hour.
2) If Helen's average speed on this trip had been 8 kilometers per hour greater, it would have taken her 1 hour less.

OA C

[Jay@ManhattanReview]

GMAT Prep

How many hours did it take Helen to drive from her house to her parent's house?

1) Helen's average speed on this trip was 72 kilometers per hour.
2) If Helen's average speed on this trip had been 8 kilometers per hour greater, it would have taken her 1 hour less.

OA C

Say the distance from Helen's home to her parent's house is d kilometers.

We have to get the value of d.

Let's take each statement one by one.

1) Helen's average speed on this trip was 72 kilometers per hour.

=> time taken by Helen = d/72 hours.

Can't get the value of d. insufficient.

2) If Helen's average speed on this trip had been 8 kilometers per hour greater, it would have taken her 1 hour less.

Only this information can't information. Insufficient.

(1) and (2) together

=> d/72 - d/80 = 1

Since this is a linear relationship, we will get a unique value of d. Sufficient.

Since this is a DS question, we need not get the actual value.

The correct answer: C

Hope this helps!

-Jay
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[coursetakers]

d=rtd=rt , where dd is the distance covered, rr rate and tt time. Question: t=?t=?

(1) r=72r=72. Clearly not sufficient.
(2) (r+8)(t−1)=d(r+8)(t−1)=d. Clearly not sufficient.

(1)+(2) r=72r=72 and (r+8)(t−1)=d(r+8)(t−1)=d --> 80(t−1)=d80(t−1)=d --> d=80(t−1)=rt=72td=80(t−1)=rt=72t --> 8t=808t=80 --> t=10t=10. Sufficient.

Answer is C