Developmental Mathematics Student Workbook Level 3 Ones Subtraction Concepts and Basic Facts Author:L. George Saad Grade 2, Semester 1 - Level 3. Ones. Subtraction Concepts and Basic Facts Level 3. Ones. Subtraction Concepts and Basic Facts covers the concept and symbol of subtraction using the 36 subtraction facts with ones and reading and writing subtraction number sentences. Level 3 introduces the concept of zero using the 19 addition facts and 19 subtra... more »ction facts with zero.« less