Confusing graphs/relations Q

[sandi]

During the holiday Bruce decides to go on a long drive up the highway, to no destination in particular. He gets in his car at home, Point O, and drives due north at a constant speed of 65km/h
ON the same day, Bruce's friend Sally also decides to go on a long drive on the same stretch of highway. Sally's house, point S, is 385 kms directly south of Bruce's home. Sally drives north at a constant speed of 100km/hr
Both Brue and Sally leave on their drives at exactly the same time.

QUESTION: Write a linear equation describing the distance Sally travels north, with respect to Bruce's home?
ANSWER: -385+100(hours)=Distance

Can anyone explain why it is negative 385 and not positive? I though it would be positive because Sally is moving towards Bruce's house....

[Conic]

I think it considers north as positive and south as negative. It's initially 385km south and then travels 100km/h north.

[JEStarr]

I think it assumes that Point O has coordinates (0,0) and therefore is Sally is 385 km away from that point in thesouth direction (assuming that the positive y values is north, and negative y values is south), then sally's house would be at point (0,385)