Oscar wants to train for a marathon. He plans to add 2/3 of a mile each week until he reaches a 20-mile run. How many weeks before the marathon should he start training if he has already run 2 miles? Let's think first. Chain of thought:
Oscar needs to increase his maximum running time from 20 - 2 = 18 miles. Oscar will need a total of 18 / ( 2 / 3 ) = 27 weeks to prepare.
Therefore, the answer is 27.