What is the difference between directly propotional and inversely proportional?

Two variables, X and Y, are directly proportional if Y = cX where c is a constant. They are inversely proportional is Y = k/X where k is a constant.

In directly proportional the two variable vary in the same "direction". So, if one increases, the other increases.

In inversely proportional, the two variable vary in opposite "directions". So, if one increases, the other decreases.