As L and W are not given and we can assume it to 100 for both for easy calculation
Initial L = 100, W = 100 , Area = 10000
Final L = 125, W = 80 , Area = 10000

Another method:
Assume both for 100 now 25% increment = 5/4 and 20% decrement = 4/5
Area = 5/4 * 4/5 = 1 so no change

Note:
We need to assume proper numbers for faster calculation.
For if the question was asked for increment of 11.11% == 10/9 in fraction and decrement of 9.09% == 10/11 in fraction then our assumption values of L and W should be 99x and 99y. Now final values will be L = 110x and W = 90y