Question 1157101
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Let n be the number of the shirts under the question.


Then you have this equation for n

    {{{720/(n-2)}}} - {{{720/n}}} = 4.


(price of one shirt in the hypothetical scenario MINUS price of one sheet in real scenario).


Simplify by dividing  both sides by 4

    {{{180/(n-2)}}} - {{{180/n}}} = 1.


Reduce to the standard quadratic equation form and get the answer n = 20.
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Solved.