Question 852182
We would only assume that the rocks will be used as one length horizontally around each garden.  Estimate this area of border this way:

Border+Garden-Garden=Border


{{{(4+1/2)(9+1/2)-4*9=BorderArea}}}
{{{4.5*9.5-36=6.75}}} square feet.


The rocks for border will be most likely in a rectangular arrangement.  Treating each rock as if it were a square, it would cover an area {{{(1/2)(1/2)}}} square foot, or 0.25 square feet.


{{{(6.75*border)/(0.25*rock)=27}}}
Each rock fits in the border 27 times.  The estimate of how many rocks for each garden is 27.