This procedure has actually been used to "weigh" astronauts in space. A 42.5-\rm kg chair is attached to a spring and allowed to oscillate. When it is empty the chair takes 1.30 \rm s to make one complete vibration. But with a person sitting in it, with her feet off the floor, the chair now takes 2.54 \rm s for one cycle.
What is the mass of the person?