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It reaches terminal velocity after falling about 10m
so we can find how long it takes to fall 10m from the equation s=ut+0.5gt^2
s=10m
ut=0 since initial velocity is zero
g=9.81
therefore t is about 1.43 s
so it reaches terminal velocity(7ms^-1) at about 1.43 s
But in the answer it says it has to be between 1.5s and 3.5s.