A migrating bird flew across a lake at an average speed of 18 meters p

Question

A migrating bird flew across a lake at an average speed of 18 meters per second. Was the distance that the bird flew across the lake less than 21 kilometers? (1 kilometer = 1000 meters)

(1) It took the bird less than 20 minutes to fly across the lake.

(2) It took the bird less than 0.4 hour to fly across the lake.

chetan2u · Accepted Answer

Hi,
the moment we are given SPEED and asked for distance, and the choices are in TIME, CHANGE speed and distance in TIME...
At a speed of 18m per sec, the bird will travel 21 kms in 21000/18 secs = 7000/6 seconds or 1167 seconds..
When can we answer the Q-
 A) If the time is less than 1167 secs, we can answer the Q as definite YES
B) If the time is greater than 1167 secs, we can answer the Q as definite NO..

lets see the statements

(1) It took the bird less than 20 minutes to fly across the lake. 
20 minutes is 1200 secs...
so time can be 1180 , ans is NO
and if time is 1110, ans is YES..
Insuff..

(2) It took the bird less than 0.4 hour to fly across the lake. 
0.4hour = .4*60 *60 secs = 1440 secs..
we can take same example as in (1)..
again Insuff..

Combined.. 
the set of examples remain..
 time can be 1180 , ans is NO
and if time is 1110, ans is YES..
Insuff..
E

Bunuel · Answer

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 Speeding, Relatively 
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Cez005 · Answer

1) Distance covered in 20min = 18x60x20=21600. Therefore, distance is <21600. Insuff.
2) 0.4h = 24min, which allows for a greater distance than 1). Insuff.
1) and 2) Insuff. No new information from using both statements.

ezzo · Answer

Can you let me know where I went wrong:
I converted Speed into Km/h so its 64.8 Km/h
Now according to first statement t<20 mins i.e less than 1/3 hrs or 0.33 hrs

Now If I calculate the distance taking the max cvalue say 0.32 * 64.8 its 20.7 which is less than 21 hence I marked A.

crza · Answer

Hi ezzo,
Statement 1 says that it took the bird less than 20 minutes(or 0.33hrs) to fly across the lake. The time could be 0.32 hours, or it could be 0.3299 hours.
If the time were 0.32 hours, as you have calculated, the distance is less than 21. However, if the time were 0.3299 hours, the distance is 21.38, which is more than 21km.

crza · Answer

In these kinds of questions, it is best to first calculate the required information to answer the question, and then compare this information with Statements 1 and 2.
For example, we are given:
s=18 m/s 
and we want to find out if total distance is less than 21km.
d>21km or 21,000m

First, convert speed to m/min to make the information comparable to statements 1 and 2.
(18 m/s) (60 s/min) = 18*6 m/min

To calculate the time,
t> 21,000/(18*6) = (7*3*100)/(9*2*3*2)  
 t could be less than 19.44 or could be more than 19.44. INSUFFICIENT.  
(2) It took the bird less than 0.4 hour to fly across the lake. 
 0.4 hour=24min. Again, t could be less than 19.44 or could be more than 19.44. INSUFFICIENT.  
 
Statement b did not really bring new information as we already know from Statement 1 that t is less than 19.44.
Thus 1&2 INSUFFICIENT.

chetan2u · Answer

hi dont complicate the calculations by taking decimals..
so t<20min or t<1/3..
thus d<t*speed or d< 64.8*1/3...d<21.6
so d can be 21.5 also and 10 also..
insuff

hope it helps

Prashant10692 · Answer

Bunuel  Do we have a separate question directory for this type of questions? atleast one such question comes in gmat always.

hellosanthosh2k2 · Answer

Question: 18t < 21000
=> t < 21000/18 secs?
=> t < 7000/6 secs?
=> t < 7000/360 mins?
=> t < 19.x mins?

Statement 1: It took the bird less than 20 minutes to fly across the lake. => t < 20 mins => not suff
Statement 2: It took the bird less than 0.4 hour to fly across the lake. => t < 24 mins => not suff

combining 1 + 2, t < 20, but still insuff to answer, t < 19.x?=> (E)

gmatapprentice · Answer

chetan2u  --wanted to understand logic of below statement and understand scenarios how it would work if we are given distance or time instead of speed.

the moment we are given SPEED and asked for distance, and the choices are in TIME, CHANGE speed and distance in TIME...

chetan2u · Answer

Hi

The main point is that we should look at choices

If the choices are given in terms of time, we should try to convert the main stem in same units as comparing the like items , be it time, distance or speed , is easy and comparing speed and time can involve some mistakes.

ShankSouljaBoi · Answer

Dear  Bunuel  ,

Please tag this question --- GMATPREP EP6

Best !!

Bunuel · Answer

________________
Done. Thank you!

GMATGuruNY · Answer

Determine the number of minutes required to travel 21 kilometers at the given rate of 18 meters per second:
 \frac{21-kilometers}{1} * \frac{1000-meters}{1-kilometer} * \frac{1-second}{18-meters} * \frac{1-minute}{60-seconds} ≈ 19.5  minutes
Since 21 kilometers can be traveled in approximately 19.5 minutes, the distance will be LESS than 21 kilometers if the time is LESS than approximately 19.5 minutes.
Question stem, rephrased:
 Was the time less than approximately 19.5 minutes?

In Statement 1, the time is less than 20 minutes.
In Statement 2, the time is less than 24 minutes.
When the statements are combined, the time can be 19 minutes (in which case the answer to the rephrased question stem is YES) or 19.9 minutes (in which case the answer is NO).
INSUFFICIENT.

E

Sam2021 · Answer

We are given that speed = 18m/sec and are asked if d<21km or d<21000 m

We know that distance = speed*time. So, putting d=18*time(t), we get,
=> 18t<21000
=> t<21000/18
=> t<7000/6
=> t<1166.6

now let's look at the options
1) It took the bird less than 20 minutes to fly across the lake.
=> t<20 min
covering min to sec
=> t<20*60
=> t<1200

Not Sufficient

(2) It took the bird less than 0.4 hours to fly across the lake.
0.4 hours = 0.4*60*60 = 1440 sec
since t<1440 sec so we can't say if t<1166.6

Taking them together doesn't give us any new info. So answer is E.

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A migrating bird flew across a lake at an average speed of 18 meters p

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