Question 48559
Let the width of the margin be x cm.
The area of the printed matter can be represented by the length of the page less twice the margin (20-2x) times the width of the page less twice the margin (16-2x), and this is given as 140 sq.cm.  You can write the appropriate equation for this:
{{{(20-2x)(16-2x) = 140}}} Perform the indicated multiplication and simplify.
{{{320-72x+4x^2 = 140}}} Rewrite as:
{{{4x^2-72x+320 = 140}}} Subtract 140 from both sides of the equation.
{{{4x^2-72x+180 = 0}}} Factor out a 4 to simplify.
{{{4(x^2-18x+45) = 0}}} Apply the zero product principle.
{{{x^2-18x+45 = 0}}} Solve by factoring.
{{{(x-3)(x-15) = 0}}} Apply the zero product principle.
{{{x-3 = 0}}} and/or {{{x-15 = 0}}}
If {{{x-3 = 0}}} then {{{x = 3}}}
If {{{x-15 = 0}}} then {{{x = 15}}}

The margin is either 3 cm wide or 15 cm wide.
But if the margin were 15 cm wide, there would be no space for the printed matter because the page is only 16 cm wide and the margin of 15 cm on both sides of the page just doesn't make any practical sense...so discard the 15 cm solution.

The margin has to be 3 cm wide.