Question 77326
h = -5t^2+20t+100 
When will the ball reach a height of 80m?
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80 = -5t^2+20t+100
-5t^2+20t+20=0
t^2-4t-4=0
t=[4+sqrt(16-4*-4]/2
t=[4+sqrt(32)]/2
t=[4+4sqrt2]/2
t=2+2sqrt2 seconds
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Cheers,
Stan H.