A box contains cardboards numbered from 1 to 21 . Cardboards are taken out one by one randomly . find the probability that the numbers on the drawn cards will be in A.P.??

Its not possible to find such a probability cos there can be several A.P.s formed.

and the no. of cards nor difference is given.

if diff. is given then make several A.P.s possible and then their count (prob.) over total (prob.) draw!!!