HI Yulia,
The calculations you do will work on the entire dataset not just the selections, however, there a number of ways to get to your answer. You need to pull out a result dataset based on the selection you want. You can do that spatially or through an attribute filter.
First, if you drag the selected features to a new map > you now have a dataset based on you selection.
Now, go to the data pane and use the new result dataset you just created (based on your selection).
Open the data table from the "..." off the new dataset name and do your calculation.
Now you can place your new resulst in whatever card type you card - a summary table, graph or map.
Hope that helps.