In how many ways can 4 prizes be given to 3 boys when a boy is eligible for all prizes?

Let suppose 4 prizes be P1, P2, P3, P4 and 3 boys be B1, B2, B3

Now P1 can be distributed to 3 boys(B1, B2, B3) by 3 ways,


Similarly, P2 can be distributed to 3 boys(B1, B2, B3) by 3 ways,


Similarly, P3 can be distributed to 3 boys(B1, B2, B3) by 3 ways,


And P4 can be distributed to 3 boys(B1, B2, B3) by 3 ways


So total number of ways is 3 × 3 × 3 × 3 =81


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