Which number should replace the question mark?

A young teenaged girl was a prostitute and, for obvious reasons, kept it a secret from her Grandma. One day, the police raided a brothel and arrested a group of prostitutes, including the young girl.
The prostitutes were instructed to line up in a straight line on the sidewalk. Well, who should be walking in the neighborhood, but little old Grandma. The young girl became frantic.
Sure enough, Grandma noticed her young granddaughter and asked curiously, "What are you lining up for dear?" Not willing to let grandma in on her secret, the young girl said that some people were giving out free oranges and that she was lining up for some.
"Mmmm, sounds lovely," said Grandma, "I think I'll have some myself," she continued as she made her way to the back of the line. A police officer made his way down the line, questioning all of the prostitutes. When he got to Grandma at the end of the line, he was bewildered. "But, you're so old, how do you do it?"

Grandma replied," Oh, it's quite easy sonny, I just remove my dentures and suck 'em dry."