A string is clamped at one end and the other end rests on a light, frictionless pulley with a mass hanging below it to provide a tension. The string is vibrated by a small motor with a fixed frequency which generates small amplitude vibrations. It is found that when the mass is 68.5 kg the string resonates in a mode that has a single anti-node in the middle of the string. The mass is then adjusted and it is found that for a new mass the string resonates with 10 anti-nodes. What is the mass on the end of the string when this happens?