GMAT Problem Solving (PS) | Re: Two alloys A and B are composed of two basic elements. The

Let E1 and E2 be elements one and two, respectively.

E1 (A) = 5
E2 (A) = 3
Total (A) = 8

E1 (B) = 1
E2 (B) = 2
Total (B) = 3

A in X = 4, which is half of Total (A) - so E1 and E2 in X from A are half of what they are in A.
B in X = 3, which is the same as in Total (B) - so E1 and E2 in X from B are the same as what they are in B.

E1 (X from A) = 5/2
E2 (X from A) = 3/2

E1 (X from B) = 1
E2 (X from B) =2

[E1 (X from A) + E1 (X from B)] :[E2 (X from A) + E2 (X from B)] = 5/2 + 1 : 3/2 + 2 = 3.5 : 3.5 = 1 : 1. Ans - A.
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