To use the elimination method, you have to create variables that have the same coefficient, then you can eliminate them.
We are given

we can use distribute property
Distributive property:

so, we can write it as


now, we can multiply terms
and we get
..............Answer
Its because 20 is doubling its self. Its using 100% of its number, While 40 is decreasing by 50%. Half of forty is 20. Half equals 50%.