Question

The cost for Andrea to produce several units of a certain item is $336 plus $12 per unit. She sells this item for $28 per unit.

Answer 1

Profit will be achieved when the money earned is greater than the money spent so,
336 + (n*12) = n*28  any subsequent units sold would result in profit
336 = 28n - 12n
336 = 16n
=>  n = 336/16 = 21
21 units must be sold to draw even any more would result in profit