i = 3:5
 choose(7,i)*choose(6, 5-i)
 ## 525 210  21
 sum(choose(7,i)*choose(6, 5-i))
 ## 756

dhyper(i, 7, 6, 5)
 ## 0.40792541 0.16317016 0.01631702
 sum(dhyper(i, 7, 6, 5))
 ## 0.5874126
 sum(dhyper(i, 7, 6, 5))*choose(13,5)
 ## 756