I was friends with, colleagues with, a young woman who wanted to join the Royal Navy but couldn't pass the exam due to her lack of maths. She asked me to teach her maths. I accepted because I liked her and thought it had to be simple. I'm good at maths, maybe not in this forum but by British averages.
I was not good at teaching, a totally new skill to me that I had to teach myself. This girl, this woman, couldn't even cope with long division or multiplication, and that was so basic for me that I struggled to explain it. I'd previously taught my eight year old niece to multiply in hex, I was flummoxed by this woman.
I have so much respect for primary school teachers now, back then I thought they were glorified baby-sitters. My pupil enrolled in the RN.