15. A train leaves the station traveling at 40 miles per hour. A second train leaves the station traveling
at 60 miles per hour. The second train leaves the
station 2 hours after the first train leaves. Both
trains stop traveling 4 hours after the first train
left. Together, the two trains traveled how many
miles?
A. 160
B. 200
C. 240
D. 280
E. 400
(Please solve ?)

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