7 - 11 year olds

Every day I take an afternoon stroll through a large field that has three exits at one end, which face a street. My route is like this of this drawing, parallel and perpendicular to the street.

Each exit is 5 meters wide. Between two consecutive exits there are 70 meters.

Sometimes I modify the distance to the street. In the drawing it is 3 meters. But other times it is 1, other 5, etc., always integer values, from 1 to 10.

The walk of the drawing is 235 meters (The drawing is NOT made to scale)

a-- How many meters away from the street do I have to walk so that my walk length is 250 meters (in the drawing that distance is 3 meters)?

b-- How many meters away from the street do I need to walk so that my walk length is 280 meters?