Question 831642
Are both gardens the same size and shape,
namely 5' x 8' ?
If so, when they overlap, you lose the area of 
the overlap from the sum of the areas of
the rectangles, so
Area of garden = {{{ 5*8 + 5*8 - 15 }}}
{{{ 2*5*8 - 15 = 80 - 15 }}}
{{{ 80 - 15 = 65 }}}
65 ft2
That's if the rectangles are both 5' x 8'