The ratio of copper to zinc in an alloy 'A' of 7kgs is 5:2. The ratio of the same metals in that order in another alloy 'B' of 7kgs is 3:4. If 28kg of alloy is made by mixing A and B in quantities x & y respectively so as to have the ratio of copper and zink in the ratio 1:1, then x : y is

Copper and Zinc ratio in A and B are 5:2 and 3:4.

According to question,

x+y=28...................(1)

in x quantity of A copper and zinc are as following :

copper=(5x/7) and zinc =(2x/7).

in y quantity of B copper and zinc are as following :

copper=3y/7 and zinc=(4y/7).

So,total copper=(5x/7+3y/7)

and total zinc=(2x/7+4y/7).

According to question:

(5x/7+3y/7)=(2x/7+4y/7)

or,5x+3y=2x+4y.

or,3x=y. Now put this value in (1).

4x=28 or, x=7.

So,y=28-7=21.

So,x:y=7:21=1:3.

D is correct choice.