Christa, Jada, and Yvette were swimming laps at an outdoor swimming

Question

Christa, Jada, and Yvette were swimming laps at an outdoor swimming pool. Christa planned to swim for Cm minutes at a constant speed and swim a total of CL laps. Jada planned to swim for Jm minutes at a constant speed and swim a total of JL laps. Yvette planned to swim for Ym minutes at a constant speed and swim a total of YL laps. They started swimming at the same time and stopped swimming at the same time when lightning began to occur. If Christa lost 40% of her planned swimming time, which of the 3 swimmers lost the greatest percentage of her planned laps?

1) CL = 18, JL = 25, YL = 20

2) Cm = 75, Jm = 60, Ym = 50

1) C L  = 18, J L  = 25, Y L  = 20

2) C m  = 75, J m  = 60, Y m  = 50

MartyMurray · Accepted Answer

Christa, Jada, and Yvette were swimming laps at an outdoor swimming pool. Christa planned to swim for Cm minutes at a constant speed and swim a total of CL laps. Jada planned to swim for Jm minutes at a constant speed and swim a total of JL laps. Yvette planned to swim for Ym minutes at a constant speed and swim a total of YL laps. They started swimming at the same time and stopped swimming at the same time when lightning began to occur. If Christa lost 40% of her planned swimming time, which of the 3 swimmers lost the greatest percentage of her planned laps?

1) CL = 18, JL = 25, YL = 20

2) Cm = 75, Jm = 60, Ym = 50

Since the passage says that the three swimmers started swimming at the same time and stopped swimming at the same time, we know that they all swam for the same amount of time. So, to determine which of the 3 swimmers lost the greatest percentage of her planned laps, we need a way to determine the relative percentages of their swimming they would have completed in the same amount of time.

Key to notice is that the swimmers all swam at constant speeds. So, the percentage of their planned time that they swam is equal to the percentage of their planned laps that they swam. For example, a swimmer who swam at a constant rate for half her planned time would complete half her planned laps.

1) C L  = 18, J L  = 25, Y L  = 20

This choice tells us only how many laps each planned to complete. That information does not enable us to connect their percentages completed with the amount of time they swam.

After all, they could have been swimming at the same speed or different speeds. So, even though this choice tells us that Jada and Yvette planned to complete more laps than Christa, we don't have enough information to determine what percentage of their laps they completed in the same amount of time.

For instance, if Jada and Yvette swam at the same speed as Christa, then Jada lost the greatest percentage of her planned laps since she had the most to complete and thus completed the smallest percentage of her laps in the same amount of time as the other two.

The exact numbers in that case are the following:

Christa: 40% of time lost means 40% of laps lost. 18 x .60 = 10.8 laps completed

Jada: 10.8 laps completed, (25 - 10.8)/25 = 14.2/25 = 56.8% lost

Yvette: 10.8 laps completed, (20 - 10.8)/20 = 9.2/20 = 46% lost

On the the other hand, if Jada and Yvette swam twice as fast as Christa, then Christa lost the greatest percentage of her planned laps since she was completing laps more slowly than the other two.

The exact numbers in that case are the following:

Christa: 18 x .60 = 10.8 laps, 40% lost

Jada: 2 x 10.8 = 21.6 laps, (25 - 21.6)/25 = 3.4/25 = 13.6% lost

Yvette: 2 x 10.8 = 21.6 laps, could have completed over 20 laps, 0% lost

Insufficient.

2) C m  = 75, J m  = 60, Y m  = 50

This choice gives us a way to determine the relative percentages of their swimming they would have completed in the same amount of time.

If Christa lost 40% of her planned time, then she must have swum for 60% of her planned time.

So, Christa swam for .60 x 75 = 45 minutes.

Also, since Christa lost 40% of her planned time, she lost 40% of her laps.

Since they all swam for the same amount of time, we know that Jada swam for 45 minutes as well. 45 minutes is 75% of the 60 minutes she planned to swim at a constant rate. Swimming at a constant rate for 75% of the time she planned to swim, she completed 75% of her laps.

So, Jada lost 25% her laps.

We also know that Yvette swam for 45 minutes, which is 90% of the 50 minutes she planned to swim.

So, Yvette lost 10% of her laps.

Thus, we can determine that Christa lost the greatest percentage of her planned laps.

In fact, we didn't even need to do these calculations. As soon as we saw that we had the amounts of time for which they planned to swim, we could tell that we could determine the relative percentages of their laps they completed since they all swam for the same amount of time. After, the one who planned to swim for the longest would have completed the smallest percentage of her laps.

Sufficient.

Correct Answer  B

KarishmaB · Answer

Responding to a pm: First thing to note here- there are just too many variables. So I know I cannot "work" with those. No equations, no values - GMAT doesn't afford me that much time. Which means that the question is completely conceptual. Christa's time is Cm and number of laps is Cl. In Cm minutes she plans to cover Cl laps. Similarly for other 2. The swimmers start at the same time and had to stop suddenly at the same time so their time was cut short and they swam for exactly the same time. This means the number of laps they were to do were also cut short. Question: Which of the 3 swimmers lost the greatest percentage of her planned laps? ...and here is where GMAT wins me over. They ask which swimmer lost the greatest PERCENTAGE of her planned laps. So for which swimmer were laps lost/laps planned the highest? Think about it conceptually - If I lose 50% of my time, I lose 50% of my laps. If I lose 40% of my time, I lose 40% of my laps. How many laps I was actually going to take doesn't matter. 1) CL = 18, JL = 25, YL = 20 Irrelevant till we are given how many laps they lost or how much time they lost. 2) Cm = 75, Jm = 60, Ym = 50 This is all we needed. I can say immediately that Cm lost the highest percentage of her laps because she lost the highest percentage of her time because she had planned to swim for the longest time. This statement alone is sufficient. Answer (B) In case, you want to see numerically how statement 2 works, notice this: Cm = 75 mins and she lost 40% of her time i.e. she lost 30 mins. So they swam for 45 mins all together and she lost 40% of her laps. Jm is 60 and she swam for 45 mins too. So she lost only 25% of her time (15/60) which means she lost only 25% of her laps. Ym is 50 and she swam for 45 mins too. So she lost only 10% of her time (5/50) which means she lost only 10% of her laps. Here is another interesting Math type but actually only conceptual DS question discussion: https://youtu.be/8pY6b1aYMfk It is based on the concept of another official practice question.

egmat · Answer

Here is a  detailed video solution  for this interesting question.
See how this question can be solved in the test environment using " Owning the Dataset " approach.

https://www.youtube.com/watch?v=jvPZoKncCyM

Let us know if you have any questions.

einstein801 · Answer

MartyMurray  Hi Marty, I dont think you are right here: You are assuming that they all swam the same number of laps - resulting in their time swam being sufficient to determine who lost the greatest percentage of their laps.

Lost laps: Christina: 45% ; Jada: 25% ; Yvette: 10%

If Yvette's laps were 1000, 10% of which is 100 and Christina is 100, 45% of which is 45, Yvette lost more laps.

mbaprepavi · Answer

Though Yvette lost more "number of laps", we are concerned with the percentage.

The question is asking which swimmer lost the greatest  percentage  of their planned laps. The percentage of laps lost is based on  time lost , since each swimmer swims at a constant speed. The number of laps is irrelevant because the  percentage lost  only depends on the percentage of time lost, which is the same for all swimmers, regardless of their individual planned lap numbers.

GMAT loves playing candidates in this way - Percentage vs Number / Correlation vs Proportional etc..
Ive learnt this the hard way - its extremely important to pay attention to detail
That will solve 90% of the DI question

Hope that clarifies your doubt.

wedonttalkanymo · Answer

Hi!

I chose C because we don't know if the laps could be 0. What if CL = 0, JL = 0, YL = 0? Then we don't know who lost the most percentage of their laps.

bumpbot · Answer

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Christa, Jada, and Yvette were swimming laps at an outdoor swimming

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Christa, Jada, and Yvette were swimming laps at an outdoor swimming

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