What is the context of the numerical sequences and equations?

The numerical sequences and equations appear to be part of elementary school math exercises. The descending sequence (20 down to 10) is likely for practicing number order or counting backward. The number decomposition diagrams (e.g., 9 = 2 + 7) are common tools for teaching addition and subtraction concepts, showing how a whole can be broken down into parts. The arithmetic equations ('8-6+7=9', etc.) are direct practice in calculation. The line segment measurements ('5 CM', '7 CM') indicate problems related to measurement and geometry.