Topic Details (Notes format)

How to Solve Proportions (Cross-Multiplication)

Subject: Mathematics

Book: Maths Mastery

Proportions equate two ratios, like a/b = c/d. Cross-multiplication states ad = bc. For example, if 2/5 = x/15, then 2 × 15 = 5 × x, meaning 30 = 5x, so x = 6. Proportions answer real-world questions: “If 2 liters costs ₹100, what does 5 liters cost?” or “If 3 out of 10 students prefer a subject, how many out of 50 might prefer it?” The technique is vital for scaling recipes, resizing images, or solving geometry similarity. Consistent practice ensures you can set up and solve ratio-based statements in a single, efficient step.

Practice Questions

The ratio of two numbers is 3:5, and their sum is 64. What are the numbers?

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What is the sum of the first 10 positive even numbers?

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A number is increased by 20% and then decreased by 20%. What is the net change?

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If x^2 - 5x + 6 = 0, what are the roots?

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A man spends 75% of his income and saves Rs. 600. What is his total income?

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A sum of money doubles itself in 5 years at simple interest. What is the rate of interest?

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If log(100) = 2 and log(10) = 1, what is log(1000)?

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If 2x = 16, what is the value of x?

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What is the cube of 4?

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If the perimeter of a square is 36 cm, what is the length of its diagonal?

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