\frac{ΔL}{L} = \frac{1 cm}{10 cm} = 0.1 = 10%

\frac{Δw}{w} = \frac{1 cm}{5 cm} = 0.2 = 20%

A = L w = (10 cm) (5 cm) = 50 cm^{2}

\frac{ΔA}{A} = \frac{ΔL}{L} + \frac{Δw}{w} = 10% + 20% = 30%

ΔA = (A) (\frac{ΔA}{A}) = (50 cm^{2}) (30%) = 15 cm^{2}

A = 50 cm^{2} ± 15 cm^{2}