solution:-

2) 4 zeros. when we split the 20! as prime factors we get 4 fives. and 10 is nothing but 2x5. since 20! has 4 fives and many twos. it is possible to have 4 tens.i.e. 10000. so there will be 4 zeros in the last. 

am i right?

let me start the 1st question:

1) three nos. are in A.P. and their product is a prime no. tell me the nos. 

2) how many zeros does 20! contain in its end ( no calculators please, give me the explanation)