If 40% is equal to the fraction of x by 30 what is the value of x

What is the value of x in the diagram below?

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

Which of the following illustrates the product rule for logarithmic equations? a. log2(4x)=log24/log2x b. log2(4x)=log24xlog2x c. log2(4x)=log24-log2x d. log2(4x)=log24+log2x