WAEC 2010 Mathematics · Question 25

A casting is made up of Copper and Zinc. If 65% of the casting is Zinc and there are 147g of Copper. What is the mass of the casting?

Options

A.320g
B.420g
C.520g
D.620g

Answer: B. 420g

Explanation written with AI and checked against the answer key

Analyze the question The casting contains only two metals, Copper and Zinc. We are told Zinc is 65% of the casting, and the Copper has a mass of 147 g. We need the total mass. Concept If Zinc is 65%, then Copper is the remaining percentage: Copper % = 100% − 65% = 35% Solution Let the total mass of the casting be M. 35% of M = 147 g (35/100) × M = 147 M = 147 × 100 / 35 M = 14700 / 35 M = 420 g Check: Zinc = 65% of 420 = 273 g Copper = 147 g Total = 273 + 147 = 420 g ✓ Answer: The mass of the casting is 420 g, which is option B. Tip: In "part and whole" percentage problems, always find the percentage that matches the given quantity first (here, Copper = 35%), then divide the given mass by that percentage to get the whole. Do not apply 65% to 147 g, because 147 g is not the whole.

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