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.

-0.8t^{2} + 50t + 10 = 0

t ≅ -0.199 or 62.699

so

t = 62.699 sec