This product helps students understand the concept of missing addends. Common Core aligned for first grade. Includes directions for the activity, a color and black and white version of the worksheet and monster number cards from 1-20. Students use the cards to fill the known and sum columns of the worksheet and then must solve for the missing addend. Can easily differentiate by cutting the number of cards from 1-20 to 1-10. Great for small group or math centers.