There are twenty books on a shelf, six Science books and fourteen History books. Each student goes up to the shelf, selects a book and brings it back to their desk. What is the probability that both books will be Science

The sum of two numbers is 12 and their difference is 18 what are the two numbers

If a || b and e || f , what if the value of y ?

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