Calculate the percolation rate for a sample if it took 20 minutes for 200 ml of water to percolate.

As we know,

Percolation rate =$\frac{Amount of water \left(ml\right)}{percolation time \left(min\right)}$

                              =$\frac{200 ml}{20 min}$ = 10 ml/min.    

Hence, the percolation rate of this sample is 10 ml/min.