Let's take the first volume of fifth-grade mathematics of People's Education Press as an example:

Unit 1: decimal multiplication

Unit 2: Decimal Division

Unit 3: Observing Objects

Unit 4: Simple Equation

Unit 5: Area of Polygon

Unit 6: Statistics and Possibility

Unit 7: Mathematical Wide Angle

Unit 8: General Review

Therefore, the simple equation should be in Unit 4.