A number theory problem by sudoku subbu

Number Theory Level 3

A POSTMAN HAS TO DELIVER 5 LETTERS TO 5 DIFFERENT PLACES. MISCHIEVIOUSLY HE POSTS ONE LETTER TO EACH PLACE WITHOUT SEEING WHETHER IT WAS THE CORRECT PLACE. IN HOW MANY DIFFERENT POSSIBLE WAYS COULD HE DO THIS SO THAT EXACTLY TWO OF FIVE PLACES RECEIVE CORRECT LETTERS.


The answer is 20.

Sudoku Subbu by sudoku subbu , 19, India

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

3 solutions

Ganesh Ayyappan
Jan 9, 2015

this problem shud be under @combinatorics .... and its also under-rated .... can give 50-60 points

1 Helpful 0 Interesting 0 Brilliant 0 Confused
Ajay Krisshan
Jan 7, 2015

5p2=5!/(5-2)!=20

1 Helpful 0 Interesting 0 Brilliant 0 Confused

DEARRANGEMENT THEOREM Though my method was lengthy as compared to Ajay krisshans' procedure, I hope this helps you.

Dearrangement theorem:

if "n" items are arranged, then total ways they can be rearranged so that nobody occupies assigned position is thus:

n=5 and r=3 5!/3! * (1-1+1/2 - 1/3)

an important formula

0 Helpful 0 Interesting 0 Brilliant 0 Confused

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...