I created this for my post graduate program and would like to share with anyone interested for free. In this file, you will find the first 5 introductory lessons for teaching subtraction of 3 digit numbers to Grade 3s using problem based learning, number routines, number talks, and alternative algorithms that are developmentally appropriate. This is ideal for new teachers, or experienced teachers interested in more student centred approaches to teaching mathematical computations. I have also posted similar introductory lessons for Grades 3 and 4 addition, subtraction, multiplication, and division. Take what you like from the lessons.