5. If a quantity increases as a second quantity decreases and decreases as the second quantity increases, the two quantities are said to be:

A_Directly proportional
B_Inversely proportional
C_Completely unrelated
D_Blue and red

The two quantities are said to be inversely proportional. To determine if two quantities are inversely proportional, you need to consider the relationship between their changes. If an increase in one quantity corresponds to a decrease in the other, and vice versa, then the two quantities are inversely proportional. In this case, as the second quantity decreases, the first quantity increases, and as the second quantity increases, the first quantity decreases. Therefore, the correct answer is B_Inversely proportional.

C. is the best answer. Strictly speaking, "inversely proportional" for variables x and y, means x*y = constant. There could be other relationships where one goes up while the other goes down.