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Q. Number of ways of selection of 4 letters of the word "PERFECT" is

Permutations and Combinations

Solution:

7 letters
$P \rightarrow 1 $
$E \rightarrow 2 $
$R \rightarrow 1 $
$F \rightarrow 1 $
$C \rightarrow 1 $
$T \rightarrow 1$
$2 \text { alike }+2 \text { diff. }=1 \times{ }^5 C _2=10 $
$\text { All } 4 \text { different }={ }^6 C _4=15 \Rightarrow 25$