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Quantitative Reasoning – Practice – Work with ratios!

PSA_LEVEL-UP_QNR13. A and B are 2 alloys of argentum and brass prepared by mixing metal inproportions 7:2 and 7:11 respectively. if equal quantities of the two alloys melted to form the third alloy C, the proportion of argentum and brass in C will be?

A) 5:7
B) 7:5
C) 14:13
D) 6:7

Ans:  Let us find out the ratio of Argentum to the total quantity in each case:

For alloy A, the ratio “Argentum:Total” = 7/(7+2) = 7/9
For alloy B, the ratio “Argentum:Total” = 7/(7+11) = 7/18

Since, alloy being mixed are equal in quantity,

“Argentum:Total” for the alloy C will be nothing but simple average of the “ratios”=
= 1/2 * (7/9 + 7/18)
= 21/36 = 7/12

Thus,
Argentum:Brass = 7: (12-7) = 7:5.

Alternatively,

Let us assume there are 18 units of each alloy.

In alloy A, argentum quantity = 14 units, brass quantity = 4 units

Similarly, in alloy B, argentum quantity = 7 units, brass quantity = 11 units

When we mix both the alloys,

Argentum quantity = 14+7 = 21 units, Brass quantity = 4+11= 15 units
Required ratio = 21:15 = 7:5. Option B.

PSA_LEVEL-UP_QNR13

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