The formula for area is A = bh
B = Length
H = Width
If B is 5 times longer than H...
A = 5x(x)
Now let's solve
20000 = 5(x)(x)
20000 = 5x^2
/5 /5
4000 = x^2
*square*
20 = x
The height (width) is 20. (x)
The base (length) is 100 (5x -> 5(20) -> 100)
Proof: 20 times 100 = 20000