20 represents the distance a normal eye can read and 40 represents the distance your eye read the chart.

20 represents the distance you are placed from the chart and 40 represents the distance a normal eye read the chart.

20 represents the distance you are placed from the chart and 40 represents the distance your eye read the chart.

40 represents the distance you are placed from the chart and 20 represents the distance normal eye read the chart.