Answer:
20 seconds
Step-by-step explanation:
Given :
[tex]y=-5x^2+100x[/tex]
Where x  represents time in seconds and y represents the speed in rotations per minute.
To Find : Assuming that the test was complete when the fan stopped spinning completely, how long was the test?
Solution:
Since we are supposed to find how long will it take to stop the fan completely
We assumed that  test was complete when the fan stopped spinning completely
i.e. y = Speed = 0
So, Substitute y = 0 in the equation   [tex]y=-5x^2+100x[/tex]
⇒[tex]0=-5x^2+100x[/tex]
⇒[tex]5x^2=100x[/tex]
⇒[tex]5x=100[/tex]
⇒[tex]x=\frac{100}{5}[/tex]
⇒[tex]x=20[/tex]
Thus it took 20 seconds to stop the fan
Hence the test was 20 seconds long.
