In measuring the sides of a rectangle, one side is taken 5% in excess and the other 4% in deficit. Find the error percent in the area, calculate from the those measurements.

let x and y be the sides of the rectangle then 

correct area = $\frac{105}{100}x\times \frac{96}{100}y$ 

Error% =  $\frac{504}{500}xy-xy=\frac{4}{500}xy$ %

$\frac{4}{500}xy\times \frac{1}{xy}\times 100=\frac{4}{5}=0.8$