\textbf{159-} A projectile is launched in vacuum from a height of 100 meters above the ground with initial speed $V_0$ at angle $\alpha$ above the horizontal. If the vertical component of the initial velocity is $20\,\dfrac{\text{m}}{\text{s}}$, how many seconds after launch does it pass through a height of 40 meters above the ground? $\left(g = 10\,\dfrac{\text{m}}{\text{s}^2}\right)$
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(1) $2$ \hfill (2) $4$ \hfill (3) $6$ \hfill (4) $8$
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