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Question

If tr denotes the number of 11 function from {x1,x2...xr} to {y1,y2,..yr} such that f(xi)yii=(1,2,3..r) then t4 equals

A
9!
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B
11C211
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C
9
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D
None of these
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Solution

The correct option is B 9
This is equivalent to having r letters and corresponding r envelopes. The letters are placed such that no letter goes into the corresponding envelope i.e. all letters are placed wrongly.
Here, there are 4 letters and 4 envelopes.
Thus, the number of ways = 4!(12!13!+14!)=9
Hence, (C) is correct.

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