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.