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
