What is the value of 64 1/3

Can some one explain this i got sent this. is this a threat i’m scared

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

The ph level of a blueberry is 3.1 what is the hydrogen-ion concentration [h+] for the blueberry