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

[swerve]

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 has been 8 kilometers per hour greater, it would have taken her 1 hour less.

The OA is C

Source: GMAT Prep

[Ian Stewart]

Statement 1 is clearly insufficient. Using Statement 2, maybe her speed was 8 km/hr, and the total distance was 16 km, so took 2 hours. Then if her speed was 8 km/hr greater, so was 16 km/hr, she would have saved an hour. But we know there must also be a solution where her speed was 72 km/hr, because the two Statements must be logically consistent, and in that case the time will be much greater than 1 hour, so we've found two possible answers, and Statement 2 cannot be sufficient alone.

Using both Statements, if it took her t hours, the distance was 72t, from Statement 1, and 80(t-1) from Statement 2. These are equal, as Statement 2 tells us, so we get a simple equation in one unknown that we can clearly solve, and the answer is C.