Calculate the number 7800

An old man calls his son and says, "Listen, your mother and I are getting divorced. Forty-five years of misery is long enough."

"Dad, what are you talking about?" the son screams.

“We can't stand the sight of each other any longer,” he says. "I'm sick of her face, and I'm sick of talking about this, so call your sister and tell her," and he hangs up.

Now, the son is worried. He calls his sister. She says, "Like hell they’re getting divorced!" She calls their father immediately. "You’re not getting divorced! Don't do another thing. The two of us are flying home tomorrow to talk about this. Until then, don't call a lawyer, and don't file papers. DO YOU HEAR ME?” She hangs up the phone.

The old man turns to his wife and says, "Okay, they’re both coming for Christmas and paying their own airfares.