Becoming automatic with the 4 operations reduces the mental labor of problem solving. As students progress through the grades, knowing the sums, differences, products, and quotients of one-digit numbers allows students to work with bigger numbers and complete mutli-step problems, without being weighed down by counting. In grades 2 and below, students should begin by memorizing addition and subtraction facts within 20. In grades 3 and above, students should "memorize the multiplication table," or be able to recall multiplication and division facts from memory.