Question

How many numbers greater than $6\times {10}^5$ can be formed from the digits 5, 6, 7, 8 and 9 when (a) repetition not allowed (b) repetition allowed.

Answer 1

No repetition 4.5! (beginning with 6, 7, 8 or 9)
Repetition ${4.6}^5-1$
This 1 corresponds to the number $6\times {10}^5=600000$ as it is not greater than $6\times {10}^5$.