We are given with the variable cost which is:
q = -20s + 400
The selling price is 's'. So, the profit can be represented by:
P = qs - q(12)
Subsituting:
P = (-20s + 400)s - 12 (-20s + 400)
P = -20s^2 + 640s - 4800
To optimize this, we must differentiate the equation and equate it to zero, so:\
dP/ds = -40s + 640 = 0
Solving for s,
s = 16
So, the selling price should be $16 to optimize the yearly profit.
Answer:
(B)
Step-by-step explanation:
Volume of fluid in the tank =1000 gallons
Initial Amount of Salt in the tank, A(0)= 30 pounds
Incoming brine solution of concentration 2 pounds of salt per gallon is pumped in at a rate of 4 gallons per minute.
Rate In=(concentration of salt in inflow)(input rate of brine)

The resulting mixture is pumped out at the same rate, therefore:
Rate Out =(concentration of salt in outflow)(output rate of brine)

Therefore:
The rate of change of amount of salt in the tank,

Perpendicular lines are like a plus symbol,
but intersecting lines intersect in any way.
The radius is half of the diameter.
9/2=4.5
The radius is 4.5 feet.
Hope this helps!