Illustrative MathematicsVaries

This task describes an extended instructional sequence whose purpose is to use the 1-100 number square (also called a hundreds chart) to support understanding of adding and subtracting ten from a two-digit number. The key conceptual point, that the digit in the tens place is increases or decreases by 1, is connected with the process of counting, arranged in a way that reinforces place value. In evaluating the sum of a number and ten by counting ten spaces to the right, the student will see that in adding a ten they always move to the next row. In their final observation, they should notice that the result has one additional ten and the same number of ones.