H2 maths p&c

• quailmaster

  5 Apr 12, 10:13

  Three letter are selected at random from the word pioneer . Find the probability that e selection does not contain the letter E. 

  Ans is 2/7 

  What i did:

  Total number of ways to choose 3 letters without an restriction

  Case 1: both Es selected 5C1

  Case 2: all 3 letters unique 6C3

   

  Number of ways to choose 3 letters without E 5c3

   

  Probability 5c3 /  (5c1+ 6c3 )

   

  Is there something wrog with my reasoning ?

• wee_ws

  5 Apr 12, 12:00

  Hi,

    You missed out a third possibility - no Es, whose number of ways = 5C3.

    Number of ways without restriction could directly be calculated, as 7C3.

   

    Thanks.

  Cheers,
  Wen Shih