Explanation:

Given, quantity of the alloy = 200 gm.
If Copper : Zinc = 2 : 3
Then quantity of Copper = 2/5 x 200 = 80 gm
And quantity of Zinc = 3/5 x 200 = 120gm
As per the  question,
80 + x /120 = 3/2
⇒ x = 100 gm
So  The quantity of Copper to be added is 100gm