In this lesson we study the order of operations – but only with a limited set of operations. The math problems (expressions) in the lesson may include parenthesis, multiplication, addition & subtraction, but no division or other operations. This is because students are studying this after they have learned multiplication, but not yet division, in 3rd grade.
The reason I like teaching the order of operations in the early grades is so that students get exposed AND get to practice WRITING expressions (we call them "math sentences") with several operations – and thus they get used to problems the involve more than one operation. Too many times, math books focus on word problems and questions with only one operation, in grades 1-3. As a result, children often don't learn to solve word problems that require more than one operation.
Understanding multiplication word problems, part 1 – video lesson
Math Mammoth Grade 3 curriculum