It's much more common than you'd think. I had a part time waitress gig, and I knew a girl my age (16 or 17) who applied for a job there. We had been friends during middle school, but at that point we were more or less acquaintances.
She did pretty well--had a pleasant personality and nobody caught her texting etc.
But she COULD NOT count back change, and therefore wasn't hired. It was well an truly uncomfortable watching her stare blankly at the wall, holding crumpled dollars bills in her hands while she laboriously tried to count in her head. She couldn't do it.
Don't get me started on fast food. Half of the managers couldn't even count change. There were dozens of times the entire drive thru halted because someone cashed out wrong and could not, for the life of them, figure out the correct change without the aid of the register.
Just sad. :(