Calculate : `1-2+3-4+5-6+7-8++19-20`

Calculate : 1-2+3-4+5-6+7-8+9-10

Calculate: 1-2+3-4+5-6+7-8+...+49-50.

Calculate: 1-2+3-4+5=6+7-8++49-50.

Estimate the arithmetic mean of the following data : 5, 6, 7, 8, 8, 10 10, 10, 12, 12, 13, 19,19,19,20. Verify your answers by actual calculations.