If two numbers, a and b, are to be chosen from a set of 4

Question

If two numbers, a and b, are to be chosen from a set of 4 consecutive integers starting with 1 and a set of three consecutive even integers starting with 4, respectively, what is the probability that b/a will not be an integer?
 
(A) 1/6 
(B) 1/4
(C) 1/3 
(D) 1/2 
(E) 2/3

eschn3am · Answer

I'm going to assume it's B/A and give that a shot.

A: 1, 2, 3, 4
B: 4, 6, 8

Total # of outcomes: 4*3 = 12

Choose 4: only 3 makes a non-integer
Choose 6: only 4 makes a non-integer
Choose 8: only 3 makes a non-integer

3/12 = 1/4 probability of B/A not being an integer.

Sorry if this isn't actually the problem :-D

GMATBLACKBELT · Answer

A: 1,2,3,4
B: 4,6,8

b/a=i? 4/3, 6/4, 8/3

so: 1/4*1/3 = 1/12 (3) --> 1/4

B

eschn3am · Answer

Most people feel like this when they first start studying for the GMAT. It's not that the math is extremely difficult, it's how the math is being tested that's strange and new. When I first started out I spent lots of time on this forum and made sure I really understood the concepts and reasoning behind each question. In my opinion it's not enough to blindly memorize formulas for this test, you need to understand WHY a question is solved the way it's solved. This way, when the GMAT throws you for a loop, you have solid math founded in an understanding that allows you to apply it to a broad spectrum of problems...and not just on problems and problem types you've seen in the past. so keep asking questions and digging deeper on here and before long you'll be a pro! there are also great books out there to help you brush up on math skills. I love the MGMAT series for most quant topics and Veritas Project GMAT for combinatorics. You may want to give them a look as well 8-)

vscid · Answer

if the question is modified by taking out the word 'respecitively' from this question, how would one approach it?

x2suresh · Answer

{1,2,3,4}
{4,6,8}
b/a not integer 4/3,6/3,8/3
p = 3/12 =1/4

srivas · Answer

If two numbers, a and b, are to be chosen from a set of 4 consecutive integers starting with 1 and a set of three consecutive even integers starting with 4, respectively, what is the probability that b/a will not be an integer?

Answers:
(A) 1/6
(B) 1/4
(C) 1/3
(D) 1/2
(E) 2/3

Answers:
(A) 1/6
(B) 1/4
(C) 1/3
(D) 1/2
(E) 2/3

Soln: 
a is from the following set {1,2,3,4}
b is from the following set {4,6,8}

Total number of ways of choosing 2 integers, one from each set is = 4* 3 = 12 ways

Now the number of possibilities where b/a is not an integer is for the following outcomes
{b,a} => {4,3},{6,4},{8,3} = 3 ways

Hence probability is = 3/12 = 1/4

ans is  B

vonshuriz · Answer

Hey suresh,

you probably meant:
b/a not integer 4/3,6/ 4 ,8/3

fortsill · Answer

OK, here's an example of how to get an easy question wrong. 
I assumed (misread) the question to mean that a and b sets are interchangeable.
Implying that a and b could be selected from either {1,2,3,4} or {4,6,8}.

I now realize that I had misread the question, but if that was the original question, then:
P(b/a is not an integer)
= P(b/a not an integer where b in {1234} and a in {468}) 
+ P(b/a not an integer where b in {468} and a in {1234})
= 11/12 + 3/12
= 7/12.

Going back to the question that was asked:
P(b/a not an integer where b in {468} and a in {1234}) 
= 3/12 
= 1/4.

TeamGMATIFY · Answer

In such problems, it is better to first write down all the cases that are in cosideration

Set A = {1, 2, 3, 4}
Set B = {4, 6, 8}

Total outcomes: 4*3 = 12

Favourable outcomes:
We need B/A, hence considering one element of the set B at a time

4: only 3 satisfies our condition (4 is divisible by 1, 2 and 4)
6: only 4 satisfies our condition (6 is divisible by 1, 2 and 3)
8: only 3 satisfies our condition (8 is divisible by 1, 2 and 4)
 
Favourable outcomes = 3
Probability = 3/12 = 1/4 
Option B

mvictor · Answer

a relatively easy question that can be solved by enumerating all the outcomes:
1st list: 1, 2, 3, 4
2nd list: 4, 6, 8

now, possible options:
4/1
4/2
4/3 - non-integer
4/4

6/1
6/2
6/3
6/4 - non-integer

8/1
8/2
8/3 - non-integer
8/4

total 12 outcomes, out of which 3 are "successful" outcomes. 3/12 = 1/4

siv · Answer

Am I the only one who interpreted the question in the following manner? Given that the first set consists of 4 consecutive integers starting with 1, I assumed the set will be something like this {10, 11, 12, 13} or {102,103,104,105} and same for the second set {404, 406, 408}. This made me lose my mind on how to go about

icanhazmba · Answer

You are not. The question is worded poorly.

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If two numbers, a and b, are to be chosen from a set of 4

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