This Application Problem is a part of the Lesson 12, Unit 4, Grade 1. Thomas has a box of paper clips. He used 10 of them to measure the length of his big book. There are 20 paper clips still in the box. Use the arrow way to show how many paper clips were in the box at first. This take apart with start unknown problem allows students to review the concept of mentally adding or subtracting 10 and using arrow notation to express their understanding.