Using disk method to find a value of x

Hey guys this problem has me stumped.

The region bounded by $\displaystyle y = \sqrt{x}$, $\displaystyle y = 0$, and $\displaystyle y = 4$ is revolved about the x-axis.

(a) Find the value of x in the interval [0,4] that divides the solid into two parts of equal volume.

(b) Find the values of x in the interval [0,4] that divide the solid into three parts of equal volume.

I suspect the easiest way would be to use the disk method; however, I'm not sure how to get an x value that would result in 2 or 3 parts of equal volume. I can guess and check for two, but there has to be an easier way.

Thanks