Jay Shri Krishna,  I am so much obsessed with 80 /20 rule. This rule says 20% of our work give 80% result and rest of 80% effort only give only 20% outcome. We do lots of activities in a day but 20% of those work are giving productive outcome. This we can relate in workout also ,the last 20 minutes of our workout strengthens our body .In the same way the 20% of our whole study makes the the basis for the whole subject. This rule brings the clarification to the doubt of a productive time .So it's worth notable to understand the logic of positive outcome. In our focus mode  we focus only on the the 20% of the period of session .This might seem overwhelming,  but if we look closely on this we can relate 80 /20 rule in every aspect of our life. There is not doubtful point to bring up on to this. Whatever we do in our life, only 20% of those efforts result positive or productive outcome.