A clothing store determines that in order to sell x shirts, the price per shirt should be p ( x ) = 100 − x dollars. Getting x shirts from the supplier costs the store C ( x ) = 1 , 600 + 20 x dollars. If the store’s revenue from selling x shirts is R ( x ) = x ⋅ p ( x ) , for what value of x will the store’s cost and revenue be equal?