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The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line

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The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line [#permalink]

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The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line segments of negligible width, but each has a different length: 1 inch, 2 inches, 3 inches, and 4 inches, respectively. If Tommy picks a stick at random from each of 3 different complete bags of Pick-Up Sticks, what is the probability that Tommy CANNOT form a triangle from the 3 sticks?

A. 11/32
B. 13/32
C. 15/32
D. 17/32
E. 19/32


Kudos for a correct solution.
[Reveal] Spoiler: OA

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The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line [#permalink]

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In order to form a triangle, the sum of two sides must be greater than the third side.
So we can choose the combination of numbers which will NOT result in a triangle.

Favorable outcomes:
1,1,4 (4>1+1): 3 ways of arriving at this (1,1,4), (1,4,1), (4,1,1) or
1,2,4 (4>1+2): 6 ways of arriving at this (1,2,4), (1,4,2), (2,1,4), (2,4,1), (4,1,2), (4,2,1) which is same as 3!
1,3,4 (4=1+3): 6 ways of arriving at this
2,2,4 (4=2+2): 3 ways of arriving at this
1,1,3 (3>1+1): 3 ways of arriving at this
1,2,3 (3=1+2): 6 ways of arriving at this
1,1,2 (2+1+1): 3 ways of arriving at this
Overall favourable outcomes: 30

Total outcomes: 4*4*4 = 64 (4 ways of choosing a stick from each of the 3 bags)


Probability that a triangle is not formed = 30/64 = 15/32
C is the correct option here.

Last edited by PrepTap on 06 May 2015, 22:50, edited 2 times in total.
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Re: The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line [#permalink]

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Bunuel wrote:
The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line segments of negligible width, but each has a different length: 1 inch, 2 inches, 3 inches, and 4 inches, respectively. If Tommy picks a stick at random from each of 3 different complete bags of Pick-Up Sticks, what is the probability that Tommy CANNOT form a triangle from the 3 sticks?

A. 11/32
B. 13/32
C. 15/32
D. 17/32
E. 19/32


Kudos for a correct solution.


Another method would be to find the probability of making triangles.

Total number of ways of picking 3 sticks = 4*4*4 = 64

To make a triangle, sum of length of two sticks should be less than the length of the third stick or length of any one stick should be greater than the difference of the lengths of other two.

Equilateral triangle: Pick the same stick from all 3 bags. Number of triangles = 4
(1, 1, 1), (2, 2, 2) etc

Isosceles triangle (Only 2 sides same):
The two same sides cannot be 1 since their sum will be 2 and the third side will be either 2 or more.
If the same sides are 2 in length, the third side can be between 0 and 4 (exclusive). Third side can be 1 or 3.
If the same sides are 3 in length, the third side can be between 0 and 6 (exclusive). Third side can be 1, 2 or 4.
If the same sides are 4 in length, the third side can be between 0 and 8 (exclusive). Third side can be 1, 2 or 3.
This gives 8 triangles. Each of the 8 triangles can be selected in 3 ways e.g. (2, 2, 1) or (2, 1, 2) or (1, 2, 2)
Total number of triangles = 3 * 8 = 24

Scalene triangle (All sides different):
Any side cannot be 1 because the difference between the other two sides will be at least 1.
So the triangle must be 2, 3, 4. This can be selected in 3! = 6 ways

You can make a triangle in 34 ways so you cannot make it in 30 ways.
Probability = 30/64 = 15/32
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Re: The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line [#permalink]

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New post 04 May 2015, 04:18
Bunuel wrote:
The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line segments of negligible width, but each has a different length: 1 inch, 2 inches, 3 inches, and 4 inches, respectively. If Tommy picks a stick at random from each of 3 different complete bags of Pick-Up Sticks, what is the probability that Tommy CANNOT form a triangle from the 3 sticks?

A. 11/32
B. 13/32
C. 15/32
D. 17/32
E. 19/32


Kudos for a correct solution.


MANHATTAN GMAT OFFICIAL SOLUTION:

First, make sure that you understand the problem. Essentially, Tommy picks three line segments at random. Each of the line segments could be 1, 2, 3, or 4 inches long. Then he is going to try to form a triangle. Some of the time, evidently, he will not be able to do so. The question is this: what is the probability that he cannot form a triangle from the three segments?

Recall, from your knowledge of geometry, the so-called “Triangle Inequality”: in any triangle, each side length must be less than the sum of the other two side lengths. This is simply another way of saying that the shortest path between X and Y is a straight line. If you have a triangle linking points X, Y, and Z, then the shortest way to get from X to Y is to go straight there, rather than take the detour through Z. You can also express the Triangle Inequality this way: each side length must be more than the absolute difference of the other two side lengths.

Since there aren’t tons of options for the side lengths, let’s go ahead and start constructing cases that would fail the test.

1-1-2: These three lengths would not form a triangle, because the third side (2) should be less than the sum of the other two sides (1 + 1). Now we can count the rearrangements: there are 3 ways to rearrange 1-1-2 (in other words, Tommy could pick the 2-side first, second, or third). You can do this count manually (1-1-2, 1-2-1, or 2-1-1), or you can divide 3! by 2! (the repeats) to get 3 options.

1-1-3: Another 3 options that fail the test.
1-1-4: Another 3 options.
1-2-4: Another 6 options, because you can rearrange 3 distinct sides in 6 (= 3!) different ways.
1-3-4: Another 6 options.
2-2-4: Another 3 options.

These are all the possibilities for triples that don’t form triangles (make sure you don’t double-count). Adding up all the options, you get 3 + 3 + 3 + 6 + 6 + 3 = 30.

Finally, you have to divide by all the possible outcomes. Tommy has 4 outcomes in each bag, and he picks from 3 different bags. So he has 4 × 4 × 4 = 64 possible outcomes.

30/64 = 15/32.

The correct answer is C.
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line [#permalink]

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New post 27 Aug 2017, 02:45
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Re: The 4 sticks in a complete bag of Pick-Up Sticks are all straight-line   [#permalink] 27 Aug 2017, 02:45
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