Alternatively, he's good at his job of stonewalling under difficult circumstances, not bad at his job of math. I can totally imagine that if he'd done the sum

Alternatively, he's good at his job of stonewalling under difficult circumstances, not bad at his job of math. I can totally imagine that if he'd done the sum as instructed, he'd be facing a dressing down from his boss as to why he didn't follow the simple "not without reference materials" script. I imagine they're probably instructed to avoid doing maths under pressure in general, because that's a terrible idea for any normal person. Real life is not a maths exam. Edit: It turns out he was his own boss, but I think the above still applies.