If this is a GP, then the terms can be represented as a, ar, ar^2.
So a=x+40; ar=x+4; ar^2=x-20.
From the first two equations: ar/a=(x+4)/(x+40)=r; from the second pair of equations: r=(x-20)/(x+4). Therefore, (x+4)/(x+40)=(x-20)/(x+4). We can find x from this new equation: cross-multiply: (x+4)^2=(x+40)(x-20). Expand the brackets: x^2+8x+16=x^2+20x-800; 816=12x, so x=816/12=68. r=(x+4)/(x+40)=(x-20)/(x+4)=72/108 or 48/72=2/3.
Therefore, r=2/3 (and a=108 and x=68, making the series: 108, 72, 48).