<human>:  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?
<bot>: How many miles does Oscar need to run? ** Oscar needs to increase his maximum running time from 20 - 2 = <<20-2=18>>18 miles.
How many weeks will Oscar need to prepare? ** Oscar will need a total of 18 / ( 2 / 3 ) = <<18/(2/3)=27>>27 weeks to prepare.
#### 27