The answer is 56 m
Pendulum's period (T) is: T = 2π√(L/g)
L - length
g - gravitational acceleration
L = ?
T = 15 s
_____
π = 3.14
g = 9.8 m/s² (gravitational acceleration on the Earth)
T = 2π√(L/g)
15 = 2 * 3.14 *√(L/9.8)
15 = 6.28 * √(L/9.8)
√(L/9.8) = 15 / 6.28
√(L/9.8) = 2.39
(√(L/9.8))² = 2.39²
L/9.8 = 5.71
L = 9.8 * 5.71
L = 55.958 m ≈ 56 m
