This Fluency Activity is a part of the Lesson 32, Unit 1, Grade 1. This activity supports students in developing a strong foundation in the relationship between addition and subtraction. Give 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 + blank = 5"). Ask students to count on to solve the problem.