Data Sufficiency

BTGmoderatorDC wrote:If Mary always takes the same route to work, how long did it take Mary to get to work on Friday?

(1) It took Mary 20 minutes to get to work on Thursday.

(2) Mary's average speed on her trip to work was 25 percent greater on Thursday than it was on Friday.

OA C

Source: GMAT Prep

Target question: How long did it take Mary to get to work on Friday?

Statement 1: It took Mary 20 minutes to get to work on Thursday.
Clearly, we cannot use this information to answer the target question with certainty.
Statement 1 is NOT SUFFICIENT

Statement 2: Mary's average speed on her trip to work was 25 percent greater on Thursday than it was on Friday.
We have no information about Mary's average speed on Thursday
Statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
Let d = the distance to work
Let s = Mary's speed on THURSDAY

Statement 1 tells us that d/s = 20 minutes

Statement 2 tells us that: (Mary's speed on Thursday) = (Mary's speed on Friday) + (25% of Mary's speed on Friday)
In other words: (Mary's speed on Thursday) = (Mary's speed on Friday) + (1/4 of Mary's speed on Friday)
In other words: (Mary's speed on Thursday) = (5/4)(Mary's speed on Friday)
Multiply both sides by 4/5 to get: (4/5)(Mary's speed on Thursday) = (Mary's speed on Friday)
Since s = Mary's speed on THURSDAY, we can see that 4s/5 = Mary's speed on Friday

So, Mary's travel TIME on Friday = distance/speed = d/(4s/5)
= (d)(5/4s)
= 5d/4s
= (5/4)(d/s)
Since we already know that d/s = 20 minutes, we can replace d/s with 20 minutes
We get: Mary's travel TIME on Friday = (5/4)(20 minutes) = 25 minutes
Since we can answer the target question with certainty, the combined statements are SUFFICIENT

Answer: C

Cheers,
Brent