Alexander is riding his bicycle to school. after one mile he is a third of the way there. how much more does he have to ride

You are given
  1 mile = (1/3)*(distance to school)
Multiply this equation by 3 and you have
  3 mile = (distance to school)

You also know that
  (distance ridden so far) +(remaining distance) = (distance to school)
Subsituting given information, this is
  1 mile + (remaining distance) = 3 mile
Subtracting 1 mile from this equation gives
  (remaining distance) = 2 miles

Alexander has 2 miles more to ride.

The answer would be a