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2 Part Analysis similar to Problem Solving

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by pareekbharat86 » Mon Nov 18, 2013 4:07 pm
Each time an experiment is conducted, the probability of success is 0.4. When this experiment is repeated, the results of each repetition of the experiment are independent of one another. Let the positive integer n be the number of times the experiment must be repeated until it succeeds for the first time. The integers x and y are positive. The ratio of the probability that n = x to the probability that n = y is 27/125.

Select two numbers that are consistent with the information that is given. In the first column, select the row that shows the value of x, and in the second column, select the row that shows the value of y. Make exactly one selection for each column.

X, Y=
a.3
b.7
c.8
d.10
e. 12

OA is X=D, Y= B

Just wanted to make sure if my method was correct-

Since we are asked that the experiment was conducted until we succeeded, we can deduce that fpr as long as we conducted the experiment, the result was a failure. P(failure)= 1- P(success)= 1-0.4= 0.6

P(n=x)= 0.6*0.6*0.6*....x times= 0.6^x.
Similarly (n=y)= 0.6^y

Ratio is given-

0.6^x/0.6^y= 27/125
0.6^(x-y)= (3/5)^3

Equating the powers, we get x-y= 3 (since 3/5=0.60)

Therefore among the given options, [spoiler]X= 10 [/spoiler]and [spoiler]Y= 7[/spoiler]
Thanks,
Bharat.
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Source: — Problem Solving |

by [email protected] » Tue Nov 19, 2013 12:41 am
Hi Bharat,

Yes, your thinking is correct. This IR question lends itself to same type of math rules that affect probability and exponent questions. The wording of the prompt ("select two numbers that are consistent with the information") implies that there's more than one correct answer to this question, but you have to select a set of answers that "fits." Well done.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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by Mathsbuddy » Tue Nov 19, 2013 9:59 am
pareekbharat86 wrote:Each time an experiment is conducted, the probability of success is 0.4. When this experiment is repeated, the results of each repetition of the experiment are independent of one another. Let the positive integer n be the number of times the experiment must be repeated until it succeeds for the first time. The integers x and y are positive. The ratio of the probability that n = x to the probability that n = y is 27/125.

Select two numbers that are consistent with the information that is given. In the first column, select the row that shows the value of x, and in the second column, select the row that shows the value of y. Make exactly one selection for each column.

X, Y=
a.3
b.7
c.8
d.10
e. 12

OA is X=D, Y= B

Just wanted to make sure if my method was correct-

Since we are asked that the experiment was conducted until we succeeded, we can deduce that fpr as long as we conducted the experiment, the result was a failure. P(failure)= 1- P(success)= 1-0.4= 0.6

P(n=x)= 0.6*0.6*0.6*....x times= 0.6^x.
Similarly (n=y)= 0.6^y

Ratio is given-

0.6^x/0.6^y= 27/125
0.6^(x-y)= (3/5)^3

Equating the powers, we get x-y= 3 (since 3/5=0.60)

Therefore among the given options, [spoiler]X= 10 [/spoiler]and [spoiler]Y= 7[/spoiler]
All true, valid and correct, so if you understand that then you could understand this:
You might spot early on that 27 and 125 are both cubes, therefore it is likely that x-y = 3.
Any thoughts on this?
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