IIM Indore 2019 Mixture Advanced Question with logical tricks An alloy P has copper and zinc in the 

An alloy P has copper and zinc in the proportion of 5: 2 (by weight), while another alloy Q has the same metals in the proportion of 3: 4 (by weight). If these two alloys are mixed in the proportion of a: b (by weight), a new alloy R is formed, which has equal contents of copper and zinc. Then, the proportion of copper and zinc in the alloy S, formed by mixing the two alloys P and Q in the proportion of b: a (by weight) is
A. 7: 9
B. 9: 7
C. 9: 5
D. 5: 9