Two variables are directly proportional when one varibale is multiplied or divided one by any number, the other variable is also multiplied or divided by that same number.

y = kx.

k is the constant of proportionality.

The relation is commonly denoted as:

y ∝ x

The weight of a product and its price are directly proportional.

If 1 pound of tomatoes cost $2, 2 pounds will cost $4 and ½ pound cost $1.

y = 2x

The graph is a straight line that passes through the origin and its slope is the constant of proportionality.