Yeah, that's absolutely fair, and it's a bit snobby of me to get all up in arms about forgetting a formula - although it is high school level where I live. But to be handed the formula, informed that there's an issue and still not fix it is the really hard part to wrap my head around, given it's such a basic formula.
I guess I'm also remembering someone I knew who got a programming job off the back of someone else's portfolio, who absolutely couldn't program to save their life and revealed that to me in a glaring way when I was trying to help them out. It just makes me think of that study that was done that suggested that there might be a "programmer brain" that you either have or you don't. They ended up costing that company a lot to my knowledge.