This Fluency Activity is a part of the Lesson 27, Unit 1, Grade 1. This activity supports the connection between subtraction and addition. The initial subtraction sentence, with the unknown number depicted as an empty box, may be written on the board as a visual support if needed. Say a subtraction sentence aloud, saying "the mystery number" for the unknown answer (e.g., "5 - 3 = the mystery number"). Call on a student to rephrase the sentence as an addition sentence (e.g., "3 + the mystery number = 5").