H=KW/x^3, where H is the heat, K is a constant of proportionality, W the amount of wood and x the distance from the fire. Let H=KW/x1^3 and H=2KW/x2^3, where x1 and x2 are the two distances, x2 being the new distance when the amount of wood is doubled. So KW/x1^3=2KW/x2^3 and 1/x1^3=2/x2^3, so x2^3=2x1^3, x2=x1cuberoot(2)=1.26x1. Therefore the camper has to sit 26% further away from the fire.
a) x2=15' so x1=15/cuberoot(2)=11.91 feet.
b) x2=20', x1=15.87 feet.
c) x2=26', x1=20.64 feet.