If Mary takes the same route to work, how long did it take Marry to get to work on Friday?
1) It took Marry 20 minutes to get to work on Thursday
2) Mary's average speed to work was 25% greater on Thursday than it was on Friday

Neither statement is enough independently --
Statement 1 tells us time on Thursday, but gives us no clue about Friday.
Statement 2 gives us a relative speed for the two days, but nothing concrete that will allow us to determine Friday's time.

Together, however:

Let's call the Rate on Friday R, and the Rate on Thursday 1.25r, or 5R/4.
Now, the time on Thursday is 1/3 (since 20 minutes is 1/3 hour) and the time on Friday is T.

We know that RT=D

Thursday: 5R/4 * 1/3 = D
Friday: R * T = D
Since both are equal to D, we can set 5R/4 * 1/3 = RT
R will cancel out of both sides, leaving us an equation that will allow us to solve for T, so together, we have sufficient data.

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