The accepted value for the specific heat of water is 4180 J/kg°C. what sources of error could account for any differences between calculated values and the accepted values.

The illuminance of a surface varies inversely with the square of its distance from the light source. if the illuminance of a surface is 120 lumens per square meter when its distance from a certain light source is 6 meters, by how many meters should the distance of the surface from the source be increased to reduce its illuminance to 30 lumens per square meter?