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Answer:
Every 10 seconds
Step-by-step explanation:
The bob crosses its midline whenever cos(2π(t-2)/20)=0.
Since cosθ=0 when θ=±π/2 + 2πn, we can find then the bob crosses its midline by solving:
2π(t-2)/20 = ±π/2 + 2πn
t-2 = ±5+20n
The solutions are when t = -3, 7, 17, 27, 37, ...
Therefore, the bob passes its midline every 10 seconds.