At starting hot-air balloon starts 6 feet.
So, at t = 0 , h = 6 feet.
Now, distance travelled per minute is 20 feet.
So, extra distance travelled in t minutes is 20t feet.
Now,
Total height = Staring height + increase in height w.r.t time.
h = 6 + 20t
Therefore, the height h in feet as a function of time t in minutes is h = 6 + 20t .
Hence, this is the required solution.
