To make things worse what question is Mildred unable to answer( and then she laughs about it) page 42-43 TWB

Mildred is unable to answer the question about where she and Montag met.

He clarified it. "The first time we ever met, where was it, and when?" "Why, it was at --" She stopped. "I don't know," she said. He was cold. "Can't you remember?" "It's been so long." "Only ten years, that's all, only ten!"