The rate at which anything occurs usually involves some measurable quantity over time.  Take speed for instance.  Speed is distance over time and to solve any data sufficiency question involving speed, you need to know 2 out of 3 of these variables: either speed and time to find distance, or distance and time to find speed or speed and distance to find time.

Let’s start with a speed-distance-time question

What distance did Marty drive?
(1) Wendy drove 15 miles in 20 minutes.
(2) Marty drove at the same average speed as Wendy.

The question is asking for distance, so the statement you need has to provide some information relating to Marty’s speed and time.

Statement (1) gives you the distance and time (and hence speed if you need it) of some other person – Wendy.  Statement (2) tells you that Marty drove at Wendy’s speed.   Thus the two statements together give you Marty’s speed, BUT tell you nothing about Marty’s time.  Hence the statements are insufficient.

Here’s another question: if Danielle ran a race at a constant speed, at what time did she finish?
(1) Danielle started the race at 8:00 a.m.
(2) At 9:30 a.m. Danielle was halfway through the race, and at 10:00 a.m., she was ²/₃ of the way through the race.

To know what time she finished, you need some data regarding the distance Danielle ran and the time she took to run said distance.

Statement (1) tells you what time she started, which might be important to know what time she finished but doesn’t give you data about the two things you need.  Statement (2) tells you that she took 30 minutes to run 1/6 of the race that gives you both distance and time taken to run that distance.  Thus, statement (2) is sufficient.

How about a general quantity-time-rate question?

Working at a constant rate, Keith can paint 7200 linear feet of shelving in 90 minutes. How long would it take Keith and Samantha, working together, to paint 7200 linear feet of shelving?
(1) Working alone, it takes Keith twice as long to paint the shelving as it takes Keith and Samantha working together.
(2) Both Keith and Samantha paint the shelving at same rate.

This question involves linear feet, time and rate.  It is asking for the time Keith and Samantha would take together.  Since the prompt already tells us Keith’s rate, we need to find Samantha’s rate.  Normally, I would tell you that you need to have some data regarding Samantha’s time and linear feet of shelving she can paint in that time.  But each of the statements relate Samantha’s rate to Keith’s directly.

Statement (1) tells you that Keith and Samantha take half the time as Keith alone.  So the answer is 90 minutes / 2 = 45 minutes.

Statement (2) tells you that they paint at the same rate.  So the answer would also be 90 minutes.

Clearly, each of the statements alone is sufficient.  Generally, data sufficiency questions involving rates tend to be simpler if it is not a question involving distance and time.  Because distance-speed-time formulas are supposed to be so familiar, the GMAT tends to make those questions a little more complicated.  If it is a question involving the rate of flow of a liquid or how fast someone paints or works, then it will usually just require you to look at the rates of the various people or things in question.

Please visit the Grockit forum or leave a comment here to discuss further.