Solution: Let the length of the small rectangle in the big rectangle be xm.

(x+ 100)* 0.25+(80-0.25 * 2+80)*( 100-x)+(80-0.25 * 2)x = 80 * 100

x=3 1900/8 1

100*80-(3 1900/8 1+ 100)*0.25=8000-7975/8 1-25=633200/8 1