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June 2018 p12 q3
896
A company producing salt from sea water changed to a new process. The amount of salt obtained each week increased by 2% of the amount obtained in the preceding week. It is given that in the first week after the change the company obtained 8000 kg of salt.
(i) Find the amount of salt obtained in the 12th week after the change.
(ii) Find the total amount of salt obtained in the first 12 weeks after the change.
Solution
(i) The amount of salt obtained each week forms a geometric progression with the first term \(a = 8000\) kg and a common ratio \(r = 1.02\). The amount of salt obtained in the 12th week is given by the formula for the \(n\)-th term of a geometric progression:
\(a_n = a imes r^{n-1}\)
Substituting the values, we have:
\(a_{12} = 8000 imes (1.02)^{11}\)
\(a_{12} \approx 9950 \text{ kg}\)
(ii) The total amount of salt obtained in the first 12 weeks is the sum of the first 12 terms of the geometric progression. The sum \(S_n\) of the first \(n\) terms of a geometric progression is given by: