\textbf{169.} A simple pendulum completes 40 full oscillations in 72 seconds. How should we change the length of the pendulum so that it completes 45 full oscillations in the same time? $\left(g = \pi^2\,\dfrac{\text{m}}{\text{s}^2}\right)$
(1) Decrease by 9\,cm. \qquad (2) Increase by 9\,cm.
(3) Decrease by 17\,cm. \qquad (4) Increase by 17\,cm.
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