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Take total no of participants as x
given- every particiapnt walks km equal to total active participants
so for first 3 months
total distance = x^2

for next 6 months
total participant = x/2
so distance = x/2 x x/2 = x^2 /4
and multiply by 6 for 6 months = 3x^2/2

For last 3 months
particiapnts = x/4
so distance = (x/4)^2 x 3 = 3x^2/16

Adding all
3x^2 + 3x^2/2 + 3x^2/16 = 10800
3x^2 (1 + 1/2 +1/16) =10800
3x^2 x 25/16= 10800
x^2 = 10800x16 / 25x3
x^2 = 2304
x= 48

So E in answer
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At the beginning of a yearlong walking challenge, a group of friends begins tracking the total distance walked by the group. In each month, every active participant walks and records a number of kilometers equal to the number of active participants in the challenge during that month. After the first 3 months, half of the participants leave the challenge, and after the next 6 months, half of the remaining participants leave. Any participant who leaves stops walking and recording distances for the challenge, but the distance that participant recorded earlier still counts toward the group’s total. If the group records a total of 10,800 kilometers during the year, how many participants were in the challenge at the beginning?

A. 12
B. 24
C. 30
D. 40
E. 48

let total participants be x
number of km walked is equal to number of active participants

first 3 months total distance will be
3*x^2
next 6 months distance will be when half of people leave
6 * ( x^2/4 ) ; 1.5*x^2
later half of particiants leave for balance 3 months
3 * x^2/16 = 3*x^2/16
total 1 year km walked 10,800
sum of all year
3*x^2 + 1.5*x^2 + 3*x^2/16 = 10800
solve for value of x we get x = 48

OPTION E ; 48 is correct
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first 3 months, n^2 + n^2 + n^2 = 3n^2

next 6 months = (n^2/4) * 6

next 3 months = (n^2/16) * 3

3n^2 + 3/2(n^2) + (3/16)n^2 = 10800

= 75n^2 / 16 = 10800

n = 48
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At the beginning of a yearlong walking challenge, a group of friends begins tracking the total distance walked by the group. In each month, every active participant walks and records a number of kilometers equal to the number of active participants in the challenge during that month. After the first 3 months, half of the participants leave the challenge, and after the next 6 months, half of the remaining participants leave. Any participant who leaves stops walking and recording distances for the challenge, but the distance that participant recorded earlier still counts toward the group’s total. If the group records a total of 10,800 kilometers during the year, how many participants were in the challenge at the beginning?

A. 12
B. 24
C. 30
D. 40
E. 48


 


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Number of participants at the start of the year = 4x

First 3 months, # of kms = 4x * 4x *3 = 48x^2
4th through 9th month = 2x * 2x * 6 = 24x^2
10th through 12th month = x * x * 3 = 3x^2

75x^2 = 10800

x^2 = 144

x = 12

4x = 48

Option E
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Let us assume at the start of year we have x particpants, thus in one month total km logged will be x^2.
and in first 3 months it will be 3x^2.
Then for next six months participants are half thus x/2 particpants and total km logged in a month will be (x/2)^2.
and in next 6 months it will be 6(x/2)^2.
Similarly last 3 months , total will be 3(x/4)^2
Therefore total km will be
3x^2 + 6(x/2)^2 + 3(x/4)^2 = 10800
solving this we get x = 48. Ans.
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i am going with the option e.
months 1-3 = 48 people walk 48km each for 3 months = 3x48^2 = 6912
months 4-9 = 24 people, 24 km each for 6 months = 6x24^2 = 3456
months 10-12 = 12 people 12 km each for 3 months = 3x12^2= 432, which all sums upto 10,800 and by plugging in 48 gives the exact answer
a incorrect - gives much less distance as no. of walkers is too small
b incorrect - also too small, total distance would be 1/4 of what 48 produces.
c incorrect - still too small, squared effect participants makes the total fall short.
d incorrect - close but the total distance it gives is less than the actual.
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Number of participants is N

1 to 3 months N
4 to 9 months N/2
10 to 12 months N/4

Each participants walks number of KM equal to number of participants

For first 3 months, each month they walk N x N km + N^2. For 3 months 3N^2

Next 6 months, each month they walk, (N/2)^2 = N^2/4. For 6 months 6 x N^2/4 = 3N^2/2

For last 3 months, (N/4)^2 = N^2/16. For 3 months 3 x N^2/16 = 3N^2/16

Therefore. 3N^2 + 3N^2/2 + 3N^2/16 = 10800

Factor out 3N^2.

3N^2 ( 1 + 1/2 + 1/16) = 10800

3N^2 ( 16 + 8 + 1 ) / 16 = 10800

3N^2 x 25/16 = 10800

75/16 x N^2 = 10800

75 x N^2 = 172800

N^2 = 172800/75 = 2304

N = 48

Answer E 48
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At the beginning of a yearlong walking challenge, a group of friends begins tracking the total distance walked by the group.

In each month, every active participant walks and records a number of kilometers equal to the number of active participants in the challenge during that month.

After the first 3 months, half of the participants leave the challenge, and after the next 6 months, half of the remaining participants leave.

Any participant who leaves stops walking and recording distances for the challenge, but the distance that participant recorded earlier still counts toward the group’s total.

If the group records a total of 10,800 kilometers during the year, how many participants were in the challenge at the beginning?

Let the initial number of active participants be 4x.

For first 3 months:
The number of km walked by each active participant = 4x km
Total km walked by the group = 3(4x)(4x) = 48x^2

For next 6 months:
The number of active participants = The number of km walked by each active participant = 2x
Total km walked by the group = 6(2x)(2x) = 24x^2

For last 3 months:
The number of active participants = The number of km walked by each active participant = x
Total km walked by the group = 3(x)(x) = 3x^2

Total km walked by the group during the whole year = 48x^ 2+ 24x^2 + 3x^2 = 75x^2 = 10,800
75x^2 = 10800
x^2 = 144
x = 12

The number of participants at the beginning = 4x = 4*12 = 48

IMO E
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Let there be X participants

X participants covered X distance each in each month for the first 3 months - so total distance 3 (X*X) = 3X^2
for the next 6 months, X/2 people left, and remaining X/2 covered X/2 distance each in each moth for the next 6 months = 6(X/2)^2
similarly X/4 for last three months = 3 * (X/4)^2

Total distance covered = 3 (X^2) + 6 {(X^2) /4} + 3 {(X^2)/16}
X^2 [ 3 + 3/2 + 3/16] = 3 (X^2) [ (16+8+1)/16] given it is equal to 10800 (2 * 10 * 10 * 6 * 9) (to make it simple)

(x^2) * 3 * 25 = 16 * 6 * 10 * 10 * 2 * 9

X^2 = 16 * 2 * 2 * 2 * 2 * 9
X = 4 * 2 * 2 * 3 = 48
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Forming equation can take a bit of time, so I would smartly choose the options-

Since the total distance is 10800 km, I would first choose the highest number of participants with a zero- 40

First 3 months, each participant walks 40 km, so 40 participants walk 40 kms for 3 months= 40*40*3

Next 6 months= 20*20*6

Last 3 months= 10*10*3

Total lm- 4800+2400 + 300= 7500

This is less than 10800, so the only option that should make up the 10800 km is Option (E) 48
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initial participants=N
first 3 months:3N^2
next 6 months:6(N/2)^2=3/2N^2
Last 3 months:3(N/4)^2=3/16N^2

(3+3/2+3/16)N^2=10800
75N^2/16=10800
N^2=2304
N=48
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Answer: E) 48

Within the first three months, there are x participants and x*x=x^2 km ran per month = 3x^2
After the first three months, there are 1/2x participants remaining and over the next 6 months they run 6*(x/2)^2 = 3x^2/2 km
After the first nine months, there are 1/4x participants remaining and over the next 3 months they run 3*(x/4)^2 = 3x^2/16 km

In total, they run 75x^2/16 km. Now we solve for x knowing they ran 10800km:
75x^2/16 = 10800
x = sqrt(2304) = 48
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The correct answer is E. 48

Let's assume that the number of participants at the beginning of the challenge were x.
kilometers walked by everyone will also be x.
total distance= x*x=x^2
total distance in 3 months= 3x^2

participants after 3 months= x/2
kms walked= x/2*x/2= x^2/4
total distance for 6 months= 6*x^2/4= 1.5x^2

For final 3 months
participants= x/4
distance walked= x^2/16
total distance in 3 months= 3*x^2/16= 0.187x^2

total sum of distance, given in the question is 10,800.
3x^2+1.5x^2+3x^2/16= 10800
(48x^2+24x^2+3x^2)/16= 10800
75x^2/16= 10800
75x^2= 172800
x^2= 2304
x=48
Hence our answer.
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We know that:

No. of KMs = No. of Active Participants

First 3 months: No. of Participants = x and No. of KMs walked = x ,i.e., x^2 per month or 3[x^2]

Months 4-9: No. Participants are halved = 1/2x X 1/2x = x^2/4 per month or 6[x^2/4]

Months 10-12: No. of Participants are reduced further by half preceding or by a quarter of the total, i.e., 1x/4

x/4 X x/4 = x^2/16 per month or 3[x^2/16]

Total = 3[x^2] + 6[x^2/4] + x^2/16 = 10,800
x^2 = 2,304
x = 48
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I considered total number of participants in the beginning as 20x.

Then adjusted the participant number per month as per total distance traveled by each participant.

Month Belownumber of
participants
distance traveled
per month
Total distance traveled
in that period (in km)
1 to 3 months20x20x20x * 20x * 3
3 to 9 months10x10x10x * 10x * 6
9 to 12 months5x5x5x * 5x * 3

With this we will have equal to 10800 km

400x^2 * 3 + 100x^2 * 6 = 25x^2 * 3 = 10800
on solving this x = 12/5

now I had put value of x in total number of participants, 20x = 20 * 12/5 = 48 ANSWER OPTION E
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At the beginning of a yearlong walking challenge, a group of friends begins tracking the total distance walked by the group. In each month, every active participant walks and records a number of kilometers equal to the number of active participants in the challenge during that month. After the first 3 months, half of the participants leave the challenge, and after the next 6 months, half of the remaining participants leave. Any participant who leaves stops walking and recording distances for the challenge, but the distance that participant recorded earlier still counts toward the group’s total. If the group records a total of 10,800 kilometers during the year, how many participants were in the challenge at the beginning?

For (1-3) months
Active participants= n
Each participant walks n km per month.
Total per month= n*n= n^2
For 3 months = 3n^2

For (4-9) months.
Half leave, remaining active participants= n/2
Total per month= (n/2)(n/2) = (n^2)/4
For 6 months= 6(n^2 /4) = (3n^2)/2

For (10-12) months.
Half of remaining leave again. Remaining active participants= n/4
Total per month= (n^2)/16
For 3 months= (3n^2)/16

Total distance covered in 12 months = 10,800
(3n^2)+(3n^2)/2 +(3n^2)/16 = 10800
48n^2 +24n^2 +3n^2 =172800
75n^2 = 172800
n^2 =2304
n= 48

E
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Let initial no. of participants = x
Then, distance covered by each participant (first 3 months) = x
Total distance (per month for first 3 months) = x^2
Months 1-3 distance = 3x^2 ... (a)

For months 4-9:
Participants = x/2
Distance per participant = x/2
Total distance per month = (x^2)/4
Months 4-9 distance = 6(x^2)/4 ... (b)

For months 10-12:
Participants = x/4
Distance per participant = x/4
Total distance per month = (x^2)/16
Months 10-12 distance = 3(x^2)/16 ... (c)

Adding (a), (b) and (c) will give total distance of 10,800:

3x^2 + 6(x^2)/4 + 3(x^2)/16 = 10,800

=> x^2 = (10,800 x 16)/75
= 3600 x 16/25

=> x = 60 x 4/5

=> x = 48 (Option E)

Bunuel
At the beginning of a yearlong walking challenge, a group of friends begins tracking the total distance walked by the group. In each month, every active participant walks and records a number of kilometers equal to the number of active participants in the challenge during that month. After the first 3 months, half of the participants leave the challenge, and after the next 6 months, half of the remaining participants leave. Any participant who leaves stops walking and recording distances for the challenge, but the distance that participant recorded earlier still counts toward the group’s total. If the group records a total of 10,800 kilometers during the year, how many participants were in the challenge at the beginning?

A. 12
B. 24
C. 30
D. 40
E. 48


 


This question was provided by GMAT Club
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