WAEC Mathematics 2010 Paper

Concept:The casting contains only Copper and Zinc, so their percentages add up to $100\%$. The given mass of Copper represents $35\%$ of the total mass.Explanation:Total percentage of the casting is $100\%$.Percentage of Zinc is $65\%$.Therefore, percentage of Copper is:$$100\% - 65\% = 35\%$$It is given that the mass of Copper is $147$ g.This $147$ g is $35\%$ of the total mass of the casting.Using the percentage formula:$$\text{Percentage of Copper} = \frac{\text{Mass of Copper}}{\text{Mass of Casting}} \times 100\%$$$$35\% = \frac{147}{\text{Mass of Casting}} \times 100\%$$Rearranging:$$\text{Mass of Casting} = \frac{147}{35} \times 100$$$$\text{Mass of Casting} = 4.2 \times 100 = 420 \text{ g}$$Answer:The mass of the casting is $420$ g, which is option B.