Let R=revenue, C=cost, Q=quantity sold, and S=selling price
R=S*Q
C=12*Q
R-C must be greater than or equal to 0
SQ-12Q≥0
(S-12)(200000-5000S)≥0
From this equation we see that S must be at least 12.
Expanding this equation and dividing it by 1000 give'
5S2 - 140S + 2400 and the derivative is 10S-140 which means the maximum profit will occur when S=14.