Mr. Ofori

Mr. Ofori starts a job with an annual salary of 6400, which increases by 240 every year. After working for eight years, Mr. Ofori was promoted to a new post with an annual salary of 9500, which increased by 360 yearly. Find
I. Mr. Ofori's salary in the fifteenth year is service
ii. Mr. Ofori's total earnings at the end of the fifteenth year of service

Final Answer:

a₁₅ =  11660
s₁₅ =  131980

Step-by-step explanation:

$$\begin{gathered} a_1=6400 \\ {d}_1=240 \\ {a}_n=a_1+(n-1)\cdot d \\ {a}_8=a_1+(8-1)\cdot d_1=6400+(8-1)\cdot 240=8080 \\ {a}_9=9500 \\ {d}_2=360 \\ {a}_{15}=a_9+(15-9)\cdot d_2=9500+(15-9)\cdot 360=11660 \end{gathered}$$

$$\begin{gathered} s_1=\frac{a_8+a_1}{2}\cdot 8=\frac{8080+6400}{2}\cdot 8=57920 \\ {s}_2=\frac{a_{15}+a_9}{2}\cdot 7=\frac{11660+9500}{2}\cdot 7=74060 \\ {s}_{15}=s_1+s_2=57920+74060=131980 \end{gathered}$$

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