The measured value of the length of a simple pendulum is 20cm with...

Time period of a simple pendulum is given by, $T = 2\pi\left(\dfrac{l}{g}\right)$.

$⇒ g = \dfrac{4{\pi}^{2}l}{{T}^{2}}$

Therefore, fractional error will be

$\dfrac{\Deltag}{g} = \dfrac{\Deltal}{l} + \dfrac{2\DeltaT}{T}$

$= \dfrac{0 . 2}{20} + 2\left(\dfrac{1}{40}\right)$

$= \dfrac{1 . 2}{20}$

Percentage change $= \dfrac{1 . 2}{20} \times 100 = 6$