What is the most buyers of the earphones would have been willing to pay in all for that quantity?

Explanation

1. 01

   It is the area under the demand curve: $\int_0^{6}(50 - x^{2})\,dx = \left[50x - \frac{x^{3}}{3}\right]_0^{6}$.

2. 02

   $300 - 72 = 228$.