Two alloys A and B contain gold and copper in the ratios of 2:3 and 3:7 by mass, respectively

Q. Two alloys A and B contain gold and copper in the ratios of 2:3 and 3:7 by mass, respectively. Equal masses of alloys A and B are melted to make an alloy C. The ratio of gold to copper in alloy C is 

(A) 5:10

(B) 7:13

(C) 6:11

(D) 9:13

Ans: 7:13

Sol:

Alloy A contains Gold and Copper.

Let 2x:3x

Which is same as 4x:6x

Alloy B contains Copper 3x:7x

As masses of Alloy A is equal to Alloy of mass B.
i.e

Gold Copper

i.e Alloy 1 4x : 6x⇒10 x (mass)
Alloy 2 3x : 7x⇒10 x (mass)