Bayes Theorem

Let , then we observe that and ends in 7, 9, 3 and 1, respectively.
Thus, ends in 7, 9, 3 or 1 according as i is of the form , respectively.
If is the sample space, then
is divisible by , if
(i) is of the form and n is of the form or
(ii) is of the form and n is of the form or
(iii) is of the form and n is of the form or
(iv) is of the form and n is of the form
Thus, number of favourable ordered pairs

Hence, required probability