(2) Calculate the slope a with the first point and the second point, and then calculate the intersection point b of the Y axis to obtain:
y 1 = 4x； y2= 15-5x
(3) When we met, y 1=y2, and then we got x=5/3, minus one hour's late departure, and walked for 40 minutes.
(4) y 1-y2= plus or minus 4 from the topic.
Solution, x1=11/9; X2= 19/9 is within a reasonable range. So when the distance is 44/9 and 76/9 kilometers, the two teams are 4 kilometers apart.