Rachel is indeed a nimrod when it comes to stressing out. If Mark hadn't been there, she'd still be freaking out.
Rachel: "You know I'm not as smart as you." So, she's just a girl, she's not as smart as Brendon, and she bursts into tears because she can't do things under pressure. Uck. It's a worst of Rachel triple play.
I feel bad for Mark that he's beating himself up over screwing up the decimal point the first time, because the noise alone would have kept me from being accurate with the math, especially after a nineteen hour bus ride--though I was pleasantly surprised by those buses. I was picturing your standard Greyhound or worse. They were actually pretty nice, except for the one where the window blew out.