I'm going to assume that by "reel" you mean the paper roll itself and that the weight of the empty inner core is negligible.
If my assumptions are correct, then the diameters of the core and the full reel are irrelevant to solving the question.
The weight of paper divided by the thickness in GSM gives you the area of the paper remaining. Divide that in turn by the width of the reel and you get the linear meters on the roll. To convert to linear feet, you use the conversion factor of about 3.28084 per meter.
If you put this all together with your own figures, you get
L = (W / .15 / .85 ) 3.28084
Where L is the linear feet remaining on the roll and W is the weight of the roll in kilograms. You can simplify this (within rounding) to L = 25.7321 W.
Stephen La Rocque>