This Instructional Activity is a part of the Lesson 18, Unit 4, Grade 7. Have students answer questions in this example independently. Reconvene as a class to share out and model solutions. All of the 3-letter passwords that can be formed using the letters A and B are as follows: AAA, AAB, ABA, ABB, BAA, BAB, BBA, BBB. What percent of passwords contain at least two B's?