Chris: Hi Dad, I'll be at the train station in about 15 minutes, can you pick me up?
Dad: No son I'm sorry I have to work but I'll tell mom
Chris: Okay
Dad: She'll be there in 20 minutes, please wait for her 
Chris: Okay no problem
Chris' father can't pick him up from the train station because he's working, but Chris' mother will arrive in 20 minutes and she will drive him home.