The length l of a simple pendulum is about 0.5m, and can be measured within 1mm. what accuracy is required in the measurement of periodic time T of 100 oscillations if the error in l and T are to produce equal percentage in the calculation of acceleration due to gravity (g)

The length L measurement relative error is 1/500 = 0.2%
g = L*(200 pi)^2/T^2
The relative error in T should be 0.1% if its contribution to total error equals the relative error in L.