Composition percentages in a ternary alloy: A metallic alloy contains 20 g copper, 20 g tin, and 40 g silver. What is the percentage of tin in the alloy (by mass)?

Introduction / Context:
Percentage composition by mass is a staple concept in mixtures and alloys. You find the total mass of the mixture and then compute the share of a component as a percentage of this total. The arithmetic is simple but the concept is widely applicable in chemistry and aptitude tests.

Given Data / Assumptions:

• Mass of copper = 20 g.
• Mass of tin = 20 g.
• Mass of silver = 40 g.
• All quantities are additive; no mass loss.

Concept / Approach:
Percentage of a component = (component mass / total mass) * 100. First, compute the total mass. Next, divide the tin mass by the total mass, and then multiply by 100 to convert to percent.

Step-by-Step Solution:

Verification / Alternative check:
The alloy is composed in the ratio Cu:Sn:Ag = 1:1:2. The tin fraction is 1 part out of 4 total parts ⇒ 1/4 = 25%, confirming the arithmetic.

Why Other Options Are Wrong:
20% or 30% result from miscounting parts; 40% confuses tin with silver; 10% would imply tin is the smallest fraction by far, which it is not.

Common Pitfalls:
Forgetting to sum all components to find the total, or mixing up which component’s mass is being asked for.

Final Answer:
25%