As the water is leaking uniformly out of the bucket.

 

So the mass of the bucket is varying uniformly w.r.t height 'h'

 

If we plot the curve for force (along y-axis) versus height (or displacement along x-axis). Then area under the curve is the work done

 

Here area under the curve will be = (1/2) base*altitude

 

or W = (1/2)*(20m)*(20g-10g)N = 100g Joules = 1000 Joules