area=legnth times width
width=1/5
area=1/5 times width
area=4
4=1/5 times legth
multiply both sides by 5/1 to clear fraction (since 1/5 times 5/1=5/5=1)
remember ou can do anything to an equaiton as long as you do it to both sides
20=1 times legnth
20=legnth
answer is legnth==20 m