\\\\(T=\\\\) Time period of one oscillation\\\\(t=\\\\) Time period of 100 oscillationFor 100 oscillation, \\\\(t=100\\,T\\\\)Take ln and differentiate.\\\\(\\dfrac{\\Delta t}{t}=\\dfrac{\\Delta T}{T}=\\dfrac{2}{90}\\,\\,\\,.....\\,\\,(1)\\,\\\\)Time period of one oscillation\\\\(T=2\\pi \\sqrt{\\dfrac{L}{g}}\\\\)\\\\(g=4{{\\pi }^{2}}\\dfrac{L}{{{T}^{2}}}\\\\)Take ln on both side and differentiate\\\\( \\dfrac{\\Delta g}{g}=\\pm \\left( \\dfrac{\\Delta L}{L}+2\\dfrac{\\Delta T}{T} \\right) \\\\)\\\\( \\dfrac{\\Delta g}{g}=\\pm \\left( \\dfrac{0.1}{20}+\\dfrac{2}{90} \\right)==\\pm 2.7% \\\\)Hence, error in gravity is \\\\(\\pm 2.7\\,\\\\)%