Question

Suppose a Ballon rises 250 ft during the first minute.................

Answer 1

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

`250*0.70^(n-1)ft.`

Therefore the height of the balloon in the minute n is

`\sum_{k\mathop{=}1}^n250*0.70^(k-1)ft.`

Therefore the height of the balloon in the minute 8 is:

`\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}`

Answer: Option d.