How to mange your time?
Making a weekly schedule and devote certain amount of time dayly. It can impove in your grades.Depending also varies field to study.
How to study for in 20-50 minutes chunks?
It takes time for your brain new long term memories. Can't study flat out, take 5-10 minutes breaks minimum to have your blood flowing.Physically active can playing with the dog. Do enough to get yourself pumped and to not worn out.