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Alloy A is having Gold and Copper in the ratio 2:3

&

Alloy B is having Gold and Copper in the ratio 3:7

Let's Assume we're taking 1kg of alloy A & 1kg of alloy B

∴ in alloy A Gold will be $\dfrac{2}{5}$ kg

Copper will be $\dfrac{3}{5}$ kg

in alloy B Gold will be $\dfrac{3}{10}$ kg

Copper will be $\dfrac{7}{10}$ kg

∴** In alloy C Gold to Copper ratio will be ** ${\dfrac{\dfrac{2}{5} + \dfrac{3}{10}}{\dfrac{3}{5} + \dfrac{7}{10}}}$

= ${\dfrac{\dfrac{7}{10} }{\dfrac{13}{10}}}$

= $\dfrac{7}{13}$

= **7:13 option (B)**