Why vs How (in a context)

I have a friend (Let's say Person A), and myself. He recently took a test, and told me he got a problem wrong.

Person A: "I got a problem wrong on the test."

Me: "How did you get the problem wrong?"

Person A: "Well, I didn't know how to do it."

Isn't this more of a why question? "How" relates to an action, but there's no action for "didn't know how to do it." And it makes much more sense to say "I didn't know how to do it, which is why I got the problem wrong." than "I didn't know how to do it, which is how I got the problem wrong." I wanted to correct him, but I didn't know what to say.