we also count on fingers, holding up your left thumb and saying "zero" is even more confusing shirley?
"zero" is "no fingers."
But I can see what the questioner is getting at. If you cut a cake into 10ths, it has 10 10ths. So if you asked "How many 10ths are there in a cake?" The answer would be 10. Substitute 1.5 for cake.
Yeah, but then you'd start with 1.5 cakes, so you'd have fifteen tenths.
And it's wrong anyway in the context of the first example, you've moved the goalposts. The first question is asking about units columns, the second (as you explain it) is a division question. Both are valid answers (ie, 5 or 15), but you can't just follow a units question with a mathematical division question without context, that's the sort of wooly unsolvable nonsense that ends up on Facebook. In the context of the first question, the answer to the second is "5," end of.
I'll say it again. Unless you're dealing in something other than decimal there is no way while I've got a hole in my arse that the correct answer to "how many tenths are there in 1.5" is "10."