CALCULUS the rate of change of a changing rate

31st march 2010 the above line is the definition of a differential equation an example is... the ageing process begins after a person has passed their physical peak (say 25) peoples life-spans are, on average, 75 years over a 50 year period a persons physiology breaks down faster towards the end of their life the persons ability to maintain, replace, and repair cells or resist invading cells and viruses reduces by 3% in the years 25 to 35 then by a further 7% from 35 to 45 then by 11% during the age of 45 to 55 and then by 30% from 55 to 65 and then finally it reduces a critical 50% between the age of 65 to 75 the rate of change of this changing rate over a fifty year period when written as a fraction is... one thirty third one fifteenth one ninth one third one half as a ratio it is 1:33, 1:15, 1:9, 1:3 and 1:2