THERE ARE 77 DVDS ON THE SALES RACK.
So basically, it says that there are, "23 comedies, 13 dramas, 10 science fiction movies, and 31 action movies." That's all there is on the DVD sales rack. And now, they're asking how many there are altogether, basically, by asking about how many DVDs there are in all. Not how many comedies. Not how many horror movies. And not just comedy and horror movies combined. DVDS and all. This would imply that we would have to add that all together.
23 + 13 = 36
36 + 10 = 46
46 + 31 = 77
And so there's your answer. There are 77 DVDS on the sales rack.