Problems should use the Courier font. Why? Every Courier font character uses the same amount of space. A comma is the same width as the number 5. This means that all of the numbers line up perfectly for carrying during addition and bringing the zero down in division.
For seven years I've watched my beautiful son hunched over Kumon math worksheets. He finds little enjoyment from them and I'd rather see him chasing butterflies. They've certainly helped his Math performance at school __ he's top of his class, but I wonder if the effort has been worth it in the big scheme of things. He's currently solving about 100 polynomials per day. Man, he can really solve polynomials, and fast. But for what?