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GMAT_crusher
- Junior | Next Rank: 30 Posts
- Posts: 25
- Joined: Mon Mar 31, 2008 6:09 pm
Combination probs. .. when to MULTIPLY and when to ADD ??
e.g.
To fill a number of vaccancies, an employer must hire 2 programmers from 6 applicants, and 2 managers from 4 applicants. What is the total number of ways in which she can make her selection?
Solu-
Programmers:
= (6!/(3! x 3!))
= 6x5x4 / 3x2
= 20
Managers:
= (4!/(2!x2!))
= 4x3 / 2
= 6
So the total number of ways is -
6x20 = 120
OR
6+20 = 26
??? What's the thumb-rule? easy to remember!
e.g.
To fill a number of vaccancies, an employer must hire 2 programmers from 6 applicants, and 2 managers from 4 applicants. What is the total number of ways in which she can make her selection?
Solu-
Programmers:
= (6!/(3! x 3!))
= 6x5x4 / 3x2
= 20
Managers:
= (4!/(2!x2!))
= 4x3 / 2
= 6
So the total number of ways is -
6x20 = 120
OR
6+20 = 26
??? What's the thumb-rule? easy to remember!
