A bag has 5 green marbles and 10 brown marbles

A bag has 5 green marbles and 10 brown marbles. Half of the brown marbles are made of plastic. A marble is selected at random from the bag. What is the probability that it is a brown, plastic marble? 

If an area of 300 square centimeters contains .28 ug/mL of a substance how much of the substance would be in 100 square centimeters?