Answer:
Step-by-step explanation:
Amount of brine = 100 gal
amount of salt = 80 lb
rate of water running = 4 gal / min
Let A be the amount of salt in lb in the tank at any time t.
(a) dA/dt = rate of salt in - rate of salt out
Rate of salt in = 0
rate of salt out = A x 4 / 100 = A / 25 lb/min
So, dA/dt = 0 - A/25 = - A/25 lb/min



When t = 0, A = 80 lb
So,

(b) Now, A = 40 lb
So,


take log on both the sides

t = 17.3 minutes
D is the correct answer. Since this is an equation, equation means both side are equal. So if you put each point in place of y and x and do the math on the left side, the final result of that side has to be equal to the left one. Only (-2,2) follows this statement aka its a point on this line .