Math problem: there are three points a, B and C on a straight line in order, ab = 50cm, BC = 20cm, point P is the midpoint of the line AC, find the length of the line BP

Math problem: there are three points a, B and C on a straight line in order, ab = 50cm, BC = 20cm, point P is the midpoint of the line AC, find the length of the line BP

BP=AB-AP=AB-(AB+BC)/2=50-(50+20)/2=15