The rate of change in sales S is inversely proportional to time t(t > 1) measured in weeks. Find S as a function of t if sates after 2 and 4 weeks are 200 units and 350 units respectively. A. S(t) = 50 [3 ln t/ln2 +1]
B. S(t) = 550 [3 ln t/ln3 +150]
C. S(t) = 50 [3 ln t/ln2 +11]
D. S(t) = 50 [11 ln t/ln2 +1]
E. S(t) = 550 [ln t/ln3 +1]

A. S(t) = 50 [3 ln t/ln2 +1]

Step-by-step explanation:

We are told that the rate of change in sales S is inversely proportional to time.

Thus;

dS/dt = k/t

Where k is the constant of proportionality.

So,

dS = (k/t)dt

Integrating both sides gives us;

S = (kIn t) + C

We are given that after 2 and 4 weeks are 200 units and 350 units respectively.

Thus;

S(2);

(k In 2) + C = 200 - - - (eq 1)

(k In 4) + C = 350 - - - -(eq 2)

To find k, let's subtract eq 1 from eq 2 to get;

(k In 4) - (k In 2) = 350 - 200

(k In 4) - (k In 2) = 150

k(In 4 - In 2) = 150

0.6931k = 150

k = 150/0.6931

k = 216.42

Plugging this for k in eq 1 gives;

(216.42 In 2) + C = 200

C = 200 - 150

C = 50

Thus;

S(t) = (216.42In t) + 50

Now 216.42 can also be expressed as;

150/In 2

Thus;

S(t) = ((150/In 2)In t) + 50

Factorizing out gives;

S(t) = 50 [(3 ln t/ln2) + 1]

Option A is the correct answer

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