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Let X=a1,a2,....,a6 and Y=b1,b2,b3. The number of functions f from X to Y such that it is onto and there are exactly three elements x in X such that fx=b1 is

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a
75
b
90
c
100
d
120

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detailed solution

Correct option is D

Image b1 is assigned to any three of the six pre-images in C3  6 ways.Rest two images can be assigned to remaining three pre-images in 23-2 ways (as function is onto).Hence, number of functions are C3  6×23-2=20×6=120.


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