The other tutor did not get the minimum volume of water. When we
reverse the upper two spheres the spheres do not stick up quite
as high in the cylinder.  On the left below is his solution drawn to
scale and on the right is the case when the upper two spheres
are reversed, also drawn to scale. You can see that not quite as
much water will be needed to completely submerge the three spheres
when the smallest sphere is placed between the two larger ones.

I will not solve it for you.  All you need do is follow his procedure
above to find the minimum volume of water, as it is done exactly the
same way.  Only the numbers will be different.

  

Edwin