19. Mr Ong bought a total of 156 pens and pencils. The ratio of the number of pens to the number of pencils he bought was 4:9. Each pen cost $2. How
much did Mr Ong pay for the pens?​

If you answer 2+2 you will get over 80 points

If sin(x) = 0 and cos(x) = 1, what is tan(x)?

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