Since for each succeeding minute the balloon rises 70% as much as it did in the previous minute, and in the first minute the balloon rises 250 ft, then in the minute n the balloon rises
[tex]250*0.70^{n-1}ft.[/tex]
Therefore the height of the balloon in the minute n is
[tex]\sum_{k\mathop{=}1}^n250*0.70^{k-1}ft.[/tex]
Therefore the height of the balloon in the minute 8 is:
[tex]\begin{gathered} 250*0.70^0ft+250*0.70^1ft+250*0.70^2ft+250*0.70^3ft \\ +250*0.70^4ft+250*0.70^5ft+250*0.70^6ft+250*0.70^7ft \\ \approx785.30ft \end{gathered}[/tex]
Answer: Option d.
