What do you define as proportionality it refers to a relationship; now this relationship is defined as either directly proportional or inversely proportional.
Now take case A; y is directly proportional to X - we say y varies as X varies in a progressive or retrogressive manner .The 7 serves as a constant which comes in handy when ever an equality is been introduced to an item of proportionality.
Also in case B,
As X changes y changes- x is the sole determinant of this y and hence as X increases or decreases y does so likewise.
D ; y decreases as X increases and y increases as X decrease; here the relationship of proportionality is converse . When the graph of D is plotted using any testing values we see that the graph cuts the y axis at 5 and the X axis at 5.
