The standard mix is the proportion of each product in the budget volumes.
So they were budgeting on total sales of 26,400 units.
If these, 15,840/26400 = 60% were of product A, and 10,560/26,400 = 40% were of product B.
The actual total sales were 26,700 units, and so at standard mix we would have expected Product A to be 60% x 26,700 = 16,020 units, and product B to be 40% x 26,700 = 10,680 units.