Need help pz 9.

You are at a restaurant and owe $38 for your meal. you want to leave a 20% tip. how much will your tip be?

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

Do you mind if you guys me with this question i’m on number 8