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.
Are there too many problems on the page? Some authors attempt to pack in the problems, leaving little room for students to show their work. The opposite can also be the case. Maybe there are not enough problems to accurately assess student knowledge. Are the digits on the printed page large or small? In my opinion and within reason, the numbers can never be too large. Exhibiting stress at an early age on elementary studentsí eyes from staring at small print can lead to stronger prescription lenses.