The ratio of pens to
pencils that are used by
KOA Middle School
Students is 3 : 2 If there is
a total of 160 Middle School
Students, how many of
them use pencils?

PLEASE HELP AND EXPlAIN :)

Apinhole camera is made out of a light-tight box with a piece of film attached to one side and pinhole on the opposite side. the optimum diameter d(in millimeters) of the pinhole can be modeled by d=1.9[ (5.5 x 10^-4) l) ] ^1/2, where l is the length of the camera box (in millimeters). what is the optimum diameter for a pinhole camera if the camera box has a length of 10 centimeters