10. As you know, when a lesson ends students begin to forget what they are studying. the rate at which a student forgets the material is proportional to the difference between the percentage of the material it currently remembers and the percentage minimum retention. Simon has a minimum retention rate of 50%. At the end of a lesson he had assimilated 90% of the material but a week later he was not remembers more than 70% of the material.

a) The exam takes place 2 weeks after the course. If Simon does not revise, what percentage of material will he remember on the exam?

b) How long will it be after class for Simon to remember than 55% of the material?