Direct Proportion: as one amount increases, another amount increases at the same rate. The symbol for 'direct proportional' is ∝ 

Inverse Proportion: as one value decreases at the same rate that the other increases. 

Unitary Method: this involves calculating the price per unit for each item. 

Constant: The 'constant of proportionality' is the value that relates the two amounts. y = k x where k is the constant of proportionality.