The population of the planet magrathea doubles every 40 years. in the year zero the population was 20 million. the total land area of magrathea is 200 million square kilometers. one square kilometer of land can house 200 magratheans. land that is not used for housing is used for agriculture. write down an equation whose solution is the year in which there are 30 m2 of agricultural land available for each inhabitant. solve this equation.