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