This lesson introduces the "add to with change unknown" problem type, where the action, which represents the change, is unknown (for example, "Ben has 5 pencils. He got some more from his mother. Now, he has 9 pencils. How many pencils did Ben get from his mother?"). They use counters or drawing pictures to add more to a starting quantity and learn to count on to determine the unknown part (the number it takes to reach the total).