The calculated trajectory of a firework is given by the function
s(t)=10+50t-0.8t^2
When dealing with questions like this, you need to set s(t) = 0, then solve the quadratic equation for t.
-0.8t2 + 50t + 10 = 0
t ≅ -0.199 or 62.699
so
t = 62.699 sec