Joe works on an assembly line that fills boxes with various toy trinkets. every sixth box gets a red ball and every fourt box gets a blue whistle. how often does a box get both a red and a blue whistle

The coverage of a base station of a telecommunication company forms a disk with a radius of (kilometers). let be the distance of a cellphone from a base station. assume that the location of cellphones in use are randomly uniformly distributed within the disk. calculate the mean and variance for the random variable.