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S=ut+1/2at^2
80=20t+1/2×10×t^2
80= 20t+5t^2
5t^2+20t-80=0
5{(t^2+4t-16)=0
t^2+4t-16=0
t^2+(8-4)t-16=0
t^2+8t-4t-16=0
t(t+8)-2(2t+8)
t=2,-8,-4
v= 20m/s
g= 10 m/s2
h= 80m
h = ut + 1/2 at2
80 = 0×t + 1/2 × 10 × t2
80 = 5 t2
80/5 = t2
16 = t2
so, t = 4 seconds.