Describe a process you would use to create the perpendicular bisector to a segment ab using only an unmarked straightedge and an unmarked compass

Place the compass at one end of the line segment. Set it to slightly longer than half of the line length. Draw arcs above and below the line. Place at the other end of the segment, without changing the width. Draw arcs that intersect the previous ones. The two points created form a perpendicular bisector of the line. 

She should use 12/3 to measure

The correct answer is $90