# Combinations and Permutations

## Combinations and Permutations

Permutation and combination related questions are common in PSC and Bank exams.

Before going to Combinations and Permutations, first lean about factorial.

If n is a positive integer then, factorial of n is denoted as n!.

5! = ( 1 x 2 x 3 x 4 x 5 ) = 120

4! = (1 x 2 x 3 x 4 ) = 24

Permutations are for lists of items, whose order matters and combinations are for group of items where order doesn’t matter. in other words,

• When the order of items doesn\\\'t matter, it is called as Combination.
• When the order of items does matter it is called as Permutation.

The number of permutations of n objects taken r at a time is determined by using this formula:

P(n,r)=n!/(n−r)!

Permutation: Listing your 3 favourite football team in order, from list of 10 teams. P(10,3) = 720.

The number of combinations of n objects taken r at a time is determined by using this formula:

C(n,r)=n!/((n−r)!r!)

Combination: Picking a team of 3 people from a football coaching group of 10. C(10,3) = 10!(3!(10−3)!) = 120.  Tricks and Tips for Boat and Stream type Questions

Shortcut tricks on boats and streams are one of the most important topics in exams. These are the formulas and examples on Boats and Streams (Cyclist and the wind or Swimmer and stream) questions. These examples will help you to better understand shortcut tricks on boats and streams questions.

There are multiple types of questions asked from these topics. The speed of the boat in still water and the speed of stream will give in questions, You have to find the time taken by boat to go upstream and downstream. .
The speed of the boat in up and down stream will give in question,  you need to find the average speed of the boat.
The speed of boat to go up or down the stream will give in question, you need to find speed of boat in still water and speed of stream.
The time taken by boat to reach a place in up and downstream will given in question, you need to find the distance to the place.

LINE_F...

GK പഠനത്തിനു സഹായകരമായ കോഡുകൾ

1) ഇന്ത്യയിലെത്തിയ വിദേശികൾ അവർ വന്ന ക്രമത്തിൽ = PDEF .

Portagese, Dutch,English, French.

2) വൈറ്റമിനുകൾ.

ജലത്തിൽ  ലയിക്കുന്നവ = BC.

3) കൂടുതൽ States അധികാരപരിധിയിലുള്ള ഹൈകോടതി = ഗുഹാവതി. ഏതൊക്കെ?.

States  Code = MAAN ( മിസോറം,അരുണാചൽപ്രദേശ്, അസം,നാഗാലാൻഡ്‌ ).

4) യുദ്ധങ്ങൾ അവസാനിപ്പിച്ച സന്ധികൾ.

കർണാട്ടിക് യുദ്ധങ്ങൾ...

Time and Work Problems - Shortcut Tricks and Formulas

Problems Type 1: .

A can finish work in X days. .

B can finish work in Y days.

Both can finish in Z days = (X*Y) / (X+Y). .

Problems Type 2: .

Both A and B together can do work in T days.

A can do this work in X days.

then, B can do it in Y days = (X*T) / (X-T) .

Problems Type 3: .

A can finish work in X days.

B can finish work in Y days.

C can finish work in Z days.

Together they can do work in T days = (X*Y*Z)/ [(X*Y)+(Y*Z)+(X*Z)] .

Problems Type 4: .

A can finish work in X days.

B can finish work in Y days.

A*X = B*Y.

...  #### Secretariat Assistant Syllabus 2021 #### LDC Syllabus 2022 #### Tenth Level Mains Syllabus #### Secretariat Assistant Previous Question paper #### Secretariat Assistant Syllabus 2021 #### 10th level Preliminary Exam Previous Year Question Paper #### LDC Previous Question papers #### LGS Previous Question papers #### Village Field Assistant Previous Question papers #### Preliminary Exam Answer key 2021 #### Fireman Previous year Question Papers #### UPSA Previous Question papers #### LPSA Previous Question papers   