We can solve this question using by considering the person's potential energy at different places. Let the potential energy at the ground be zero. Let the acceleration due to gravity be 9.8m/s^2. So the potential energy after reaching the top of the staircase will be mgh = 60*9.8*3.
Total work done = p.e at top - p,e at bottom = 60* 9.8 *3.
Time taken = 4 seconds.
So Power developed = work done/ time taken = 60* 9.8 *3 / 4 = 15 * 9.8 * 3 = 441 Watts.
P.S. You can simplify the above calculations by taking g = 10m/s^2.