Illumination E is proportional to luminous intensity I and inversely proportional to the square of the distance r ($E \propto I/r^2$). If the illuminations are equal, $I_1/r_1^2 = I_2/r_2^2$. Therefore, $I_1/I_2 = (r_1/r_2)^2 = (10/40)^2 = (1/4)^2 = 1/16$.