Question 4024
The perimeter of a rectangle= 2w+2l where w is the width & l is the length.

Here, width is length less 5 ft.

or w=l-5

hence replacing w by l-5 we get P= 2(l-5)+2l=50

or 50= 2l -10+2l 
or 50=4l-10
or 60=4l
or l=60/4
or l=15

hence length is 15 ft & width is 10 ft