PHILADELPHIA — The key to a deep NCAA tournament run, Mason Plumlee said after Duke advanced to the Sweet 16, is flexibility. And thanks to their early schedule, the Blue Devils learned how to be flexible. “When you look at our nonconference, we play teams that play zone, we play in pro venues, we go to the Bahamas and play in a great tournament,” Plumlee said. “There’s not much we haven’t seen at this point in the season.” But in Duke’s 66-50 win against Creighton, there was a new combination of challenges. Shots weren’t falling for the Blue Devils, who came in shooting 47 percent from the field. Ryan Kelly finished with one point. Both Plumlee and Kelly had three fouls within the first 30 seconds of the second quarter, and Plumlee had his fourth with 17:48 still left on the clock. On top of that, Duke had to slow a two-time All-American in Doug McDermott, perhaps the best scorer in the country.