Answer:
2 seconds
Step-by-step explanation:
[tex]y=-5t^2+20[/tex] (Given)
When acorn will hit the the ground, at that moment y = 0
[tex]\implies 0= -5t^2+20[/tex]
[tex]\implies 5t^2=20[/tex]
[tex]\implies t^2=\frac{20}{5}[/tex]
[tex]\implies t^2=4[/tex]
[tex]\implies t=\pm\sqrt4[/tex]
[tex]\implies t=\pm 2[/tex]
Time can not be negative.
[tex]\implies t\neq-2[/tex]
[tex]\implies t=2\:seconds[/tex]
