Open the compass it is more than half of the distance between a and b, and scribes arcs of the same radius centered at a and b. call the two points where these two arcs meet c and d. Draw the line between c and d. CD it is the perpendicular bisector of the line segment AB. call the point where CD intersects AB and E
He would have 21 before he bought the 10 stamp because 31-10=21
