With the fifteen cycles in one minute, you can first get the frequency of this pendulum which is in cycles per second or 1/4. Frequency is 1/ period so your period length is 4 you then use the period length of a pendulum equation solving for length. (equation: Period Length = 2[tex] \pi [/tex][tex] \sqrt{\frac{Length}{Gravity} } [/tex]. The length of the rope is 3.97 or about 4 meters. ( the question never asked what unit the rope is measured in, but I'm assuming it wanted meters)
