2 answers
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4-seconds.
But if you need to see the math, here you go:
Vf=Vi+at
Where
Vf=0m/s
Vi=20m/s
a=-5.0m/s
and
t=??
Solve for t:
0m/s=20m/s+(-5.0m/s^2)*t
-20m/s=(-5.0m/s^2)*t
t=-20m/s/-5.0m/s^2
t=4s