Answer
Probability = $\frac{40}{2598960}$ = $\frac{1}{64974}$
Work Step by Step
Ways to select 5 cards = $C$(52,5) = 2598960
To form a straight flush we need to have 5 cards of same suit in any of the following order
A,2,3,4,5
2,3,4,5,6
...
.
10,J,Q,K,A
10 ways $\times$ 4 =40
Probability = $\frac{40}{2598960}$ = $\frac{1}{64974}$
