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

A sum of money triples itself in 12 years at simple interest. What is the rate of interest per annum?

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What is the length of the diagonal of a square with a side length of 7 cm?

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A train 120 meters long is moving at a speed of 54 km/h. How long will it take to pass a pole?

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If the radius of a circle is doubled, what happens to its area?

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

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The angles of a quadrilateral are in the ratio 3:4:5:6. What is the largest angle?

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If a + b = 10 and ab = 21, what is the value of a^2 + b^2?

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If the length of a rectangle is doubled and the width is halved, what is the change in area?

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What is the sum of all odd numbers from 1 to 99?

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A square is inscribed in a circle with a radius of 5 cm. What is the area of the square?

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