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# Doubt on Probability Formula

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Senior Manager
Joined: 10 Jan 2013
Posts: 297
Location: India
Concentration: General Management, Strategy
GPA: 3.95

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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: 1685
Location: India
GPA: 3.01
WE: Engineering (Real Estate)

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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
_________________
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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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