assuming north as positive direction and south as negative direction of motion
y = path travelled in north direction = 20 kilometer
y' = path travelled in south direction = - 15 kilometer
distance travelled is given as
distance = d = |y| + |y'| = 20 + 15 = 35 kilometer
displacement is given as
displacement = D = y + y' = 20 + (-15) = 5 kilometer
Ratio of distance travelled to displacement is given as
Ratio = d/D = 35/5 = 7
Ratio = 7:1