Question 164163
The ticket sales peak at the equations vertex (the highest point of the quadratic equation).  The x value of the vertex can be found by the equation {{{-b/(2*a)}}}.  Your b is the coefficient of x, which is 8, and a is the coefficient of x^2, which is -.4.  Therefore the x value of the vertex is {{{-8/(2*-.4) = -8/-.8 = 10}}}.  Since your x value is the number of days, then we know daily ticket sales peak at 10 days.
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To find the number of tickets sold that day, we simply plug 10 into your equation.  So {{{N(10) = -0.4*(10)^2 + 8*10 + 12 = -.4*100 + 80 + 12 = -40 + 80 + 12 = 52}}}.  Therefore ticket sales peak at day 10, and 52 tickets will be sold that day.