Number of ways to obtain a straight flush?

Question

Straight flush: 5 consecutive ranks, all cards of the same suit.

My answer is $(13-5+1) \times 4 = 36$ but the answer my professor gives is 40. Where I got wrong?

Answer 1

The first rank can be anything between $1-10$: $$(\text{A},2,3,4,5)$$ $$(2,3,4,5,6)$$ $$\dots$$ $$(10,\text{J},\text{Q},\text{K},\text{A})$$ Thus there are $40$ straight flushes. As noted by user8734617, $(10,\text{J},\text{Q},\text{K},\text{A})$ is also considered as a "royal flush".