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Probability

Expert replies
by dvalenz » Fri Mar 29, 2013 9:37 am
If an integer n is to be chosen at random from the integers 1 to 96, inclusive, what is the probability that n(n+1)(n+2) will be divisible by 8?

Thanks
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Source: — Problem Solving |

by srcc25anu » Fri Mar 29, 2013 9:47 am
there will be 2 cases in which n * n+1 * n+2 will be divisible by 8
CAse 1: N+1 is divisible by 8. How many factors of 8 are there from 1-96
that will be: 96/8 = 12 nos {8,16,24 .... 88,96}

Case 2:N = even because if n = even, n+2 will also be even
any 2 even numbers when multiplied will contain a 8 between them. eg 2 * 4 will contain 2^3
so essentially we need to know how many enen numbers are there between 1-96 and that will be 48 Nos

Total of such selections of N = 60 Nos
Total integers between 1-96 = 96 nos
hence prob. = 60 / 96 = 5/8
hence D
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by Anju@Gurome » Fri Mar 29, 2013 10:34 am
Note that n, (n + 1), and (n + 2) are 3 consecutive integers.
If the product of three consecutive integers are divisible by 8, then the following two scenarios are possible...
  • 1. n is even --> (n + 1) is odd and (n + 2) is even --> Either n or (n + 2) will be divisible by 4
    2. (n + 1) is divisible by 8 --> n and (n + 2) are odd
Now, the above two scenarios are mutually exclusive, i.e. no three consecutive integers can be part of both the scenarios. So, there are no repetitions and we can easily calculate number of triplets in each case and add them to get the total number of triplets.

Case 1: n is even
Number of even integers between 1 ans 96, inclusive = {(96 - 2)/2} + 1 = 48

Case 2: (n + 1) is divisible by 8
n + 1 = 8a, where a ≥ 1
n = 8a - 1
8a - 1 ≤ 96
8a ≤ 97
a ≤ 12.1 implies 12 integers.

Total number of n = (48 + 12) = 60
Therefore, probability that n(n + 1)(n + 2) will be divisible by 8 = 60/96 = 5/8

The correct answer is D.
Anju Agarwal
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by GMATGuruNY » Fri Mar 29, 2013 1:09 pm
dvalenz wrote:If an integer n is to be chosen at random from the integers 1 to 96, inclusive, what is the probability that n(n+1)(n+2) will be divisible by 8?

Thanks
n(n+1)(n+2) = the product of 3 consecutive integers.

For many test-takers, the most efficient approach will be to WRITE IT OUT and LOOK FOR A PATTERN.

1*2*3
2*3*4
3*4*5
4*5*6
5*6*7
6*7*8
7*8*9
8*9*10


9*10*11
10*11*12
11*12*13
12*13*14
13*14*15
14*15*16
15*16*17
16*17*18


Each of the products in red is a multiple of 8.
The two examples above imply the following:
Of every 8 products, exactly 5 will be a multiple of 8.

Thus, the probability that n(n+1)(n+2) will be multiple of 8 = 5/8.

For a similar problem -- along with an alternate approach -- check my 2 posts here:

https://www.beatthegmat.com/probability-t116280.html
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