Answer: 5 s
Step-by-step explanation:
Given
Height of the rocket can be modelled as [tex]h=80-16t^2[/tex]
Rocket will land on earth when it's height becomes 0
[tex]\Rightarrow 80t-16t^2=0\\\Rightarrow t(80-16t)=0\\\\\Rightarrow t=0\ \text{or}\ t=\dfrac{80}{16}=5\ s[/tex]
Neglecting 0 value
Thus, rocket remains in air for 5 s.