This results in very fast answers for certain problems. It also gives them a rich understanding and intuition about numbers. For problems that aren't covered by the special cases, the general methods come into play. The Learning Kingdom's Principles Of Math Instruction Particular before general. It is important to build math intuition by learning important special cases first.
General methods are to come later.
Numbers can be mentally represented with numerals, a number line, base ten blocks, and other methods. Good teaching uses all of these representations and teaches students to switch from one to the other. Hierarchy of knowledge. Arithmetic should be broken down into as many pieces as possible. Students should not be given problems for which they haven't mastered the prerequisites.