they had to come up with the least number of coin combinations to make, say, 91 cents.
I've seen those problems, and have never had the slightest idea even how to start going about them.
I realized just now that they're saying "count back the change for a 9 cent purchase paid for with a dollar bill." And that's trivial - a penny makes 10, a nickel is 15, a dime is 25, 3 quarters makes a dollar.
I don't think it's an Asian thing. Maybe a literal-mindedness thing. The only reason it occurred to me that that's how you solve it is that DD's teacher taught them how to count back change this past week as "something most grownups don't know," and DD knew I knew how to do it from cashiering in college.
