Answer:
A) dS/dt = 2 - S/(100 + t) ; S(0) = 50
B) S(t) = (100 + t) - 5000/(100 + t)
C) 35%
Explanation:
A) Let;
t = time in minutes
S = amount of syrup
dS/dt = rate at which syrup is flowing through tank
We are told that initially, the tank contains a mixture of 450 liters of cola and 50 liters of cherry syrup.
Thus,
At t = 0
Amount of cola = 450 litres
Amount of syrup = 50 litres
Total mixture = 450 + 50 = 500 litres
We are told that a well mixed solution of cherry cola is withdrawn at the rate of 5 liters per minute.
Thus, amount of liquid in the tank at a time "t" is; 500 + 5t
We are told that cherry syrup is added at the rate of 2 liters per minute.
Thus, rate at which syrup is flowing through tank is given as;
dS/dt = 2 - 5[S/(500 + 5t)]
Factorizing out, we have;
dS/dt = 2 - 5[S/5(100 + t)]
5 will cancel out to give;
dS/dt = 2 - S/(100 + t)
Since we have 50 liters of cherry syrup initially, the initial value problem is;
dS/dt = 2 - S/(100 + t) ; S(0) = 50
B) We have our initial value problem as;
dS/dt = 2 - S/(100 + t)
Rearranging, we have;
dS/dt + S/(100 + t) = 2
Multiplying through by 100 + t gives;
(dS/dt)•(100 + t) + S = 2(100 + t)
Integrating each term with respect to t gives;
(100 + t)S = (100 + t)² + c
Divide both sides by (100 + t) to get;
S = (100 + t) + c/(100 + t)
At t = 0;
S(0) = (100 + 0) + c/(100 + 0)
S(0) = 100 + c/100
Recall earlier in our initial value problem where we stated that S(0) = 50.
Thus;
100 + c/100 = 50
Subtract 100 from both sides to givw;
c/100 = 50 - 100
c/100 = -50
c = -50 × 100
c = -5000
Thus;
S(t) = (100 + t) - 5000/(100 + t)
C) The capacity of the tank is given as 1000 liters.
Earlier, we saw that amount of liquid in the tank at a time "t" is; 500 + 5t
Thus;
500 + 5t = 1000
5t = 1000 - 500
5t = 500
t = 500/5
t = 100 minutes
We want to find What percentage of the mixture is cherry syrup when the tank is full.
Thus we will plug in t = 100 minutes into S(t) = (100 + t) - 5000/(100 + t)
Thus;
S(100) = (100 + 100) - 5000/(100 + 100)
S(100) = 200 - 25
S(100) = 175 litres
We saw earlier that total mixture initially was 500 litres.
Thus, percentage of the mixture that is cherry syrup when the tank is full is;
175/500 × 100% = 35%