If the length and width of the rectangle are $x$ and $y$ respectively, then the area of the rectangle is given by the function $f\left(x\,y\right)\=x y$. The error in measuring the area as $20$ (i.e., 4 times 5) is approximately

Indeed, the maximum error will be either $\left(4.1\right)\cdot \left(5.1\right)-20\=0.91$, or $\left(3.9\right)\cdot \left(4.9\right)-20\=0.89$. Clearly, the maximum error is $0.91$, exactly.$$