Arectangular plate has a length of (22.4 ± 0.2) cm and a width of (8.4 ± 0.1) cm. calculate the area of the plate, including its uncertainty.

Area of the rectangle = Length  x  Width
We will calculate the percentage of uncertainties. 
For a length: p 1 = ( 0.2 · 100 ) : 22.4 = 20 : 22.4 = 0.893
For a width: p 2 =  ( 0.1  · 100 ) : 8.4 = 10 : 8.4 = 1.190
p 1 + p 2 = 0.893 + 1.190 = 2.083 %
A = 22.4 · 8.4 = 188.16 +/-  2.083 %
 Uncertainty: 188.16 · 0.02083  = 3.92 
A = 188.16 +/- 3.92 cm²