Lesson: Shrinking and Expanding Gaps

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Awesome Stuff !!!
Is such practice available in SC and CR too?
gmat-admin's picture

Yes it is.
For Critical Reasoning (CR), there are practice questions categorized by question type (e.g., Assumption questions under the Assumption questions video - https://www.gmatprepnow.com/module/gmat-critical-reasoning/video/1139) and there are tons more at the bottom of https://www.gmatprepnow.com/module/gmat-critical-reasoning.

This format is difficult to apply to Sentence Correction (SC) questions. The reason for this is that almost all Sentence Correction questions test more than one concept. In fact, in most cases, they test a wide range of concepts.

So, for example, the given sentence may have an error with subject-verb agreement, and among the answer choices, one may have a verb tense error another a parallelism error, another a dangling modifier error, and so on.

Given the variety of different grammatical concepts that can be addressed in one question, it makes little sense to tackle practice questions until you’ve learned all of the required content.

For this reason, ALL of our linked practice questions are at the bottom of the SC module page: https://www.gmatprepnow.com/module/gmat-sentence-correction

Hello Brent,

Can you advise as to how to approach the below type problems using your shrink rate approach?

Eg: Sally is driving on a road to college at 30 Mph. Her friend Joe is driving on the same road 10 Miles behind her at 60Mph. Will they meet? and If they meet at what Time will they meet.

gmat-admin's picture

I'd be happy to help.

We can solve this question using some number sense.

Sally's speed = 30 mph. In other words, for every hour Sally drives, she travels 30 miles.

Joe's speed = 60 mph. In other words, for every hour Joe drives, he travels 60 miles.

So, in ONE HOUR, Joe travels 30 miles more than Sally does. This means that, EVERY HOUR, the gap between them decreases by 30 miles.

This means that in HALF AN HOUR the gap between them decreases by 15 miles, . . .

. . . and it means that in ONE-THIRD OF AN HOUR the gap between them decreases by 10 miles.

Since they started with a 10-mile gap between them, it will take 1/3 of an hour for the gap to be zero (i.e., they meet)

What if we had to take out the distance when they meet instance of time when they meet.

4/7 *42 = 24 miles
gmat-admin's picture

Yes. To be more specific, when they meet YOLANDA will have walked 24 miles (and MARK will have walked 18 miles )

A bus from city M is traveling to city N at a constant speed while another bus is making the same journey in the opposite direction at the same constant speed. They meet in point P after driving for 2 hours. The following day the buses do the return trip at the same constant speed. One bus is delayed 24 minutes and the other leaves 36 minutes earlier. If they meet 24 miles from point P, what is the distance between the two cities?

A. 48
B. 72
C. 96
D. 120
E. 192

I understand this is shrinking rate question . How to use delay and early without speed and distance
gmat-admin's picture

There are a couple of nice solutions here: https://gmatclub.com/forum/a-bus-from-city-m-is-traveling-to-city-n-at-a...

Let me know if you'd like me to clarify anything.

Hi Brent,
Can you solve this one taking several different approaches. I get that the total time between cities is 4hrs and that the distance between the buses is one hour when they leave the next day, but after that - I'm lost.
gmat-admin's picture

Hi bertyy,

Sure thing. Here's my solution: https://gmatclub.com/forum/a-bus-from-city-m-is-traveling-to-city-n-at-a...


Thanks Brent
Great step by step solution. I guess I was trying to solve this thing in one simple equation, but it turns out it is better understood (at least for me) conceptually. Unfortunately, I am not at the math god level. Darn!
gmat-admin's picture

One simple equation would be great, but this is a pretty tricky question (in the 750 range)


Hi Brent,

Two trains, X and Y, started simultaneously from opposite ends of a 100-mile route and traveled toward each other on parallel tracks. Train X, traveling at a constant rate, completed the 100-mile trip in 5 hours; Train Y, traveling at a constant rate, completed the 100-mile trip in 3 hours. How many miles had Train X traveled when it met Train Y?

(A) 37.5
(B) 40.0
(C) 60.0
(D) 62.5
(E) 77.5

My question: why can’t you solve it like this?

3x + 5x = 100
8x = 100
x = 12.5

Train X goes 62.5 miles (5 x 12.5) and train Y goes 37.5 miles (3 x 12.5).

Why are the distances switched?

gmat-admin's picture

Hi bertyy,

Can you tell me what value x represents in your solution?


The speed
gmat-admin's picture

The speed of Train X or Train Y?

Ok, I got it. Complete brain block - it was obvious once I stepped away for a while. Thank you!


In the link above, I am having hard time registering the solution. Is there any advice from you to look at it from a different perspective.
I did watch the shrinking gap problem and I get it. I believe the head start of 15 miles I throwing me off.

Based on your solution, I read that 15 Miles is the distance to catchup, however the train A is also moving (slower) and making progress. I am thinking the distance is unknown but you have used the gap to cover only as 15 miles? Please advice.

gmat-admin's picture

Link: https://gmatclub.com/forum/train-a-is-traveling-at-40-miles-per-hour-and...

When it comes to shrinking gap questions, all you need to focus on is the GAP. It doesn't matter that each train is moving at its own speed. All that matters is that the gap SHRINKS at a rate of 20 miles per hour.

In 30 minutes, train A travels 20 miles, and train B travels 30 miles.
Since train B travels 10 MILES FARTHER during those 30 minutes, the GAP between them shrinks by 10 miles during those 30 minutes.

Does that help?


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