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Question

X and Y are two alloys of copper (Cu) and zinc (Zn). Alloy X is prepared by mixing Cu and Zn in the ratio 5:4, and alloy Y is prepared by mixing Cu and Zn in the ratio 5:13 respectively. If equal quantities of alloys X and Y are melted to form a third alloy Z, then what is the ratio of Cu to Zn in Z?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
5:7

We are given two alloys, X and Y, with different proportions of copper (Cu) and zinc (Zn).

  • Alloy X contains Cu and Zn in the ratio 5:4.
  • Alloy Y contains Cu and Zn in the ratio 5:13.

When equal quantities of alloys X and Y are melted together, we need to find the ratio of Cu to Zn in the resultant alloy Z.

Step-by-Step Solution:

Let the common quantity of each alloy be 1 unit.

In alloy X, the ratio of Cu to Zn is 5:4. Therefore, in 1 unit of X, the quantities are:

  • Cu in X = \(\frac{5}{5+4} = \frac{5}{9}\)
  • Zn in X = \(\frac{4}{5+4} = \frac{4}{9}\)

In alloy Y, the ratio of Cu to Zn is 5:13. Therefore, in 1 unit of Y, the quantities are:

  • Cu in Y = \(\frac{5}{5+13} = \frac{5}{18}\)
  • Zn in Y = \(\frac{13}{5+13} = \frac{13}{18}\)

Combine both alloys to find the total Cu and Zn in the new alloy Z:

  • Total Cu in Z = \(\frac{5}{9} + \frac{5}{18} = \frac{10}{18} + \frac{5}{18} = \frac{15}{18} = \frac{5}{6}\)
  • Total Zn in Z = \(\frac{4}{9} + \frac{13}{18} = \frac{8}{18} + \frac{13}{18} = \frac{21}{18} = \frac{7}{6}\)

The ratio of Cu to Zn in Z is:

  • Cu to Zn = \(\frac{5}{6} : \frac{7}{6}\)
  • Simplifying, we find Cu:Zn = 5:7.

Therefore, the ratio of Cu to Zn in the new alloy Z is 5:7. Thus, the correct answer is 5:7.

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