How long does a mathematical pendulum have if it makes 20 oscillations in 16 seconds?

Let’s write down given:
N = 20
t = 16c
Find: l
To solve this problem, we need the formulas:
T = 2p * √ (l / g), T = t / N
Since both formulas are the formula of the period, then we can equalize them: 2n * √ (l / g) = t / N
√ (l / g) = t / (2pN)
l / g = t ^ 2 / (2Nп) ^ 2
l = (t ^ 2 * g) /(2Nп)^2=(16^2*10)/(2*3.14*20)=20.38 (m)