Question:
Write the number of arrangements of the letters of the word BANANA in which two N's come together.
Solution:
The word BANANA consists of 6 letters including three As and two Ns.
Considering both Ns together or as a single letter, we are left with 5 letters including three As.
$\therefore$ Number of arrangements of 5 things in which 3 are similar to one kind $=\frac{5 !}{3 !}=20$
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