3rd Grade Algebra Concepts and Problems - Math and Reading Help One way to approach this problem is to recall that 3 x 6 = 18, so six is the missing number. Another approach is to divide 18 by three to get six (18 ÷ 3 = 6 ). Number Properties Another algebraic concept you'll learn about in 3rd grade is the idea that there are 'properties,' or rules, that apply to all whole number operations.