Let y is directly proportional to x

=> y = kx ..........1

Again let y is inversely proportional to x

It is the same thing as: y is directly proportional to 1/x

y = k/x   ............2

Now, kx - k/x = k(x - 1/x)

                   = k(x² - 1)/x

Ex : Let the number is 5 i.e. x = 5

Now, 5k - k/5 = k(5 - 1/5)

                   = k(5² - 1)/5

                   = k(25 - 1)/5

                   = 24k/5

So, in this way, we can subtract direct and inverse proportional of a number.