Question

An alloy of copper and iron weighs $6$ kg $500$ g. The ratio of copper to iron is$5:8$. How much copper does it contain?

Answer 1

Sum of terms of the ratio = $5+8=13$
Weighs of alloy = $6$ kg $500$ g = $6.5$ kg ($500$ g = $0.5$ kg, $6$ kg + $0.5$ kg = $6.5$kg)
Quantity of copper = $\frac{5}{13}\times 6.5\mathrm{kg}=2.5\mathrm{kg}$
So alloy contains $2.5$ kg copper.