Mary of course loses potential energy. The work done by Mary as she mounted the stairs is force * distance progressed. Her weight is the force she must overawed and the space moved is 4.1 m.
Potential energy of Mary is decreased so we put negative sign
Change in energy = - m * g * h but mass = 488 / 9.8
= - (488*9.8*4.1) / 9.8
For the reason that (Weight = mass * g)
= - 19607.84 / 9.8
= - 2,000.8 Joule as she descends the stairs.
