For a simple pendulum
T = 2π*sqrt(L/g)
dT/dL = π/sqrt(L*g)
dT = π/sqrt(L*g)*dL
∆T = π/sqrt(L*g)*∆L (approximately)
∆L = sqrt(L*g)*∆T/π
Now, what is ∆T? If the clock gains 5 minutes each day, the period is too short, meaning the length is too short. The error rate is
ε = +5/(24*60) =...