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Multiples

by gmatwriter » Sun Feb 14, 2010 7:13 pm
Please Explain:

Two hundred multiples of seven are chosen at random, and 300 multiples of eight are chosen at random. Approximately what percentage of the 500 selected numbers are odd?

a. 20%
b. 25%
c. 40%
d. 50%
e. 80%

a
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by ejose » Sun Feb 14, 2010 8:09 pm
Gmatwriter --

Since this is an estimation question, we're not going to get a perfect answer and we'll have to make some assumptions. Here's how I'd attack it though:

1) Two hundred multiples of 7 ... the probability that a randomly selected one of these will be odd is 50%. (7 multiplied by even integers will yield even multiples, 7 multiplied by odd integers will yield odd multiples) Thus 100 even, 100 odd.

2) Three hundred multiples of 8. Your probability of getting an even number here is 1. 300 evens.

So you end up with 100 odd numbers, 400 even numbers, totalling 500. Your probability of selecting odd will be 100/500 = 1/5 = 20%.

The crux of this is making the assumption in point #1 that your random selections followed the probability to the letter.

-Eric
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by gmatwriter » Mon Feb 15, 2010 10:36 am
Thanks!
ejose wrote:Gmatwriter --

Since this is an estimation question, we're not going to get a perfect answer and we'll have to make some assumptions. Here's how I'd attack it though:

1) Two hundred multiples of 7 ... the probability that a randomly selected one of these will be odd is 50%. (7 multiplied by even integers will yield even multiples, 7 multiplied by odd integers will yield odd multiples) Thus 100 even, 100 odd.

2) Three hundred multiples of 8. Your probability of getting an even number here is 1. 300 evens.

So you end up with 100 odd numbers, 400 even numbers, totalling 500. Your probability of selecting odd will be 100/500 = 1/5 = 20%.

The crux of this is making the assumption in point #1 that your random selections followed the probability to the letter.

-Eric
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by shashank.ism » Mon Feb 15, 2010 10:58 am
gmatwriter wrote:Please Explain:

Two hundred multiples of seven are chosen at random, and 300 multiples of eight are chosen at random. Approximately what percentage of the 500 selected numbers are odd?

a. 20%
b. 25%
c. 40%
d. 50%
e. 80%

a
As ejose said , u can see 300 out of 500 is simply even i.e. 60%
also taking half of multiples of 7 as half as odd, 20% is even and 20% is odd
so overall 80% is may be even and 20% may be odd.
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