6 years & up. Help your children gain confidence in their math skills with this exciting Addition and Subtraction Unknown Quantities Flashcards Activity. Children will enjoy practicing addition and subtraction as they work to complete each equation. This activity promotes critical thinking, early math skills, problem solving, and algebraic thinking. Encourage your children to go over these cards with a partner and take turns practicing. Set of 162 easy to read double-sided flash cards. Cards measure 6.25"L x 3.25"W.