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There are two alloys of copper and zinc containing copper and zinc in the ratio of 1:2 and 3:5 respectively. If 12 Kgs. of the first alloy and 16 Kgs. of the second alloy are mixed and some more pure zinc is added, the ratio of copper to zinc in the resultant alloy becomes 2 : 5. How many Kgs. of pure zinc was added?

A) 6 Kgs.
B) 9 Kgs.
C) 7 Kgs.
D) 8 Kgs.

Correct Answer:



C) 7 Kgs.

Description for Correct answer:
ln alloy 1, the ratio of copper : zinc is 1 :2.
Therefore, 12 Kgs. of alloy 1 contains \( \Large \frac{1}{3} \times 12 \) = 4 Kgs. of copper and \( \Large \frac{2}{3} \times 12 \) = 8 Kgs. of zinc.
In alloy 2, the ratio of copper : zinc is 3 :5.
Therfore, 16 Kgs. of alloy 2 contains 6 Kgs. of copper and 10 Kgs. of zinc.
When these two are mixed, we get 10 Kgs. of copper and 18 Kgs. of zinc.
After that some more pure zinc is added and the resultant alloy has copper and zinc in the ratio of 2 : 5.
That is the resultant alloy has \( \Large \frac{2}{7} \)th of its weight as copper.
If copper in the alloy = 10 Kgs. = \( \Large \frac{2}{7} \)th of the total weight, then zinc in the alloy will weigh \( \Large \frac{5}{7} \)th of the total weight.
If \( \Large \frac{2}{7} \)th of the weight Is 10 Kgs., then \( \Large \frac{5}{7} \)th of the weight will be \( \Large \frac{5}{2} \times 10 \) = 25 Kgs..
18 Kgs. of zinc were present in alloy 1 and alloy 2 (8 and 10), and the final alloy contains 25 Kgs. of zinc. Hence. the balance 7 Kgs. of zinc present in the final alloy was the quantum of pure zinc added.

Part of solved Mixture and Allegation questions and answers : >> Aptitude >> Mixture and Allegation

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