Why is the product of different from the product of ?

The multiplication property of zero states that the product of any number and 0 is 0. what must be true about either a or b to make ab=0

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

When i'm with my factor 5, my prodect is 20 .when i'm with my addend 6' my sum is 10.what number am i