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# Doubt on Probability Formula

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Manager
Joined: 10 Jan 2013
Posts: 179
Location: India
Concentration: General Management, Strategy
GMAT 1: 600 Q43 V30
GPA: 3.95
Doubt on Probability Formula  [#permalink]

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07 Apr 2019, 23:54
Hi Everyone,

Could anyone please guide on the usage of the below formula?

$$P(A or B) = P(A) + P(B) - P(A and B)$$

When is the formula applicable? and any particular question in GMAT prep?

Bunuel chetan2u
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Status: It's near - I can see.
Joined: 13 Apr 2013
Posts: 1596
Location: India
Concentration: International Business, Operations
Schools: INSEAD Jan '19
GPA: 3.01
WE: Engineering (Real Estate)
Doubt on Probability Formula  [#permalink]

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08 Apr 2019, 03:08
1
saurabh9gupta wrote:
Hi Everyone,

Could anyone please guide on the usage of the below formula?

$$P(A or B) = P(A) + P(B) - P(A and B)$$

When is the formula applicable? and any particular question in GMAT prep?

Hi Saurabh,

This formula is called "Addition rule for not mutually exclusive events"

For example;

Selecting a perfect cube or an even number.

In this case, there will be n number of perfect cubes that are also even.

If I want to calculate the probability of perfect cubes or even numbers from 1 to 50 inclusive.

Total outcomes = 50.

Required outcomes : Perfect cubes = 1, 8, 27

Even numbers = 2, 4, 6, 8.....50 (Total = 25)

Required probability:

P(A or B) = P(A) + P(B) - P(A & B)

= 3(perfect cubes) + 25(even numbers) - 1(only one number is even & perfect cube = 8)

= 27

So 27/50

Hope this helps
_________________
"Do not watch clock; Do what it does. KEEP GOING."
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Joined: 21 May 2018
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Re: Doubt on Probability Formula  [#permalink]

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11 Apr 2019, 03:34
saurabh9gupta wrote:
Hi Everyone,

Could anyone please guide on the usage of the below formula?

$$P(A or B) = P(A) + P(B) - P(A and B)$$

When is the formula applicable? and any particular question in GMAT prep?

Bunuel chetan2u

Hey!

This formula is pretty much applicable throughout all domains of probability. The important part is getting P(A), P(B), P(A or B) and P(A and B) right.
These can be easily visualised by seeing some Venn Diagrams. If you ever have issues in probability, just draw these diagrams for the events and you will be able to visualise better.

Re: Doubt on Probability Formula   [#permalink] 11 Apr 2019, 03:34
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# Doubt on Probability Formula

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