We have to determine the probability of EACH card being dealt.
1) First card can be A, K, Q, J or 10 of ANY suit.
5 cards 4 suits probability = 5*4 = 20 out of 52 p = 20/52
2) Second card must be the same suit as first card AND it must be one of the 4 remaining cards. p = 4 / 51. (51 because we already have 1 card)
3) Third card can only be one of the 3 remaining cards
p = 3 / 50 (There are 50 remaining cards.)
4 & 5 we can use similar reasoning for card #4 (p = 2 / 49) and
card #5 (p = 1 / 48)
Now to get the probability of being dealt a royal straight flush, we have to multiply all 5 probabilties:
20/52 * 4/51 * 3/50 * 2/49 * 1/48 = 0.00000153907716932927 which equals
1 in 649,740 which agrees with Wikipedia
https://en.wikipedia.org/wiki/Poker_probability