An animated film shows a triangle whose dimensions vary over time (but its shape always remains a triangle). At some point in time, ` the base of the triangle is 4 cm and grows at a rate of 2 cm / s, and the height of the triangle is 3 cm and decreases at a rate of 10 cm / s. At this instant, what is the rate of change of the area of the triangle? `` Give a fully justified solution. Clearly define all the variables that appear in your solution and draw a diagram. Give your answer using the units appropriate.