"Well 12 isn't enough, so I guess I have to get 25."
Trying to be helpful, I interjected, "You could get 18. That's closer to what you wanted than 25 and it's enough."
She turned to the guy behind the counter to see if she could and he said, "No. You can get 6, 12, 25, or 50."
Still trying to be helpful, I said, "But if she gets an order of 6 and an order of 12, she'll have 18 and that's closer to what she wanted."
Ignoring me, he repeated, "You can get 6, 12, 25, or 50."
"I'll get 25 then," she said. "I'm sorry. I've never been here before; I didn't know."
ETA: Speaking of innumeracy, I'm often really dismayed to hear people - adult people, generally women - say in a totally offhand manner "Oh I never could do math" meaning by "math" basic number skills. They say it as if this were something of no importance whatsoever. I try each time (and fail) to imagine someone saying in that same "who cares" tone "Oh I never did learn how to read." Basic literacy and basic numbers skills are essential skills to be a functional adult. It's not a trivial matter to be missing either skill set.