Short Trick to bring given set of numbers in proportion

Duration: 14 min

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AI summary & chapters

AI Summary

An AI-generated summary of this video lecture.

This lecture by Yash Jain introduces ratio and proportion, focusing on a shortcut for finding the number to add or subtract from four given numbers (a, b, c, d) so that they become proportional. The instructor first defines a ratio as a comparison of two quantities and presents the general problem: what should be added or subtracted to a, b, c, d so that they are in proportion? A concrete example asks what should be added to each of 5, 9, 11, and 18. The standard algebraic method is demonstrated: introduce a variable y, write the terms as 5+y, 9+y, 11+y, and 18+y, set up the proportion (5+y)/(9+y) = (11+y)/(18+y), cross-multiply to get (5+y)(18+y) = (11+y)(9+y), expand both sides, and simplify to 90 + 23y = 99 + 20y, yielding y = 3. The instructor then introduces a shortcut formula involving |ad - bc| / (a + d) or related expressions, illustrated by subtracting the fractions 5/9 and 11/18 to obtain a difference of 3. A second example with numbers 19, 37, 47, and 86 is worked using cross-multiplication: 19 x 86 = 1634 and 37 x 47 = 1739, with the difference 105 highlighted. The lecture then shifts to the subtraction variant: what should be subtracted from each of 5, 9, 11, and 18? The terms are written as 5-y, 9-y, 11-y, and 18-y, the proportion (5-y)/(9-y) = (11-y)/(18-y) is set up, cross-multiplied and simplified to give y = -3 (or equivalently +3 in the final boxed answer). The video concludes with a thank-you slide.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The video opens with an introduction slide titled 'RATIO & PROPORTION - BY YASH JAIN', defining a ratio as 'a comparison of 2 quantities'. The instructor presents the general problem: 'What should be added/subtracted to a, b, c, d so that they are in proportion?' and then displays the specific example: 'Que: What should be added to each of 5, 9, 11, 18 to make them in proportion?' with 'each' underlined. The instructor begins writing the terms on the board, introducing a variable x (later y) to represent the unknown number being added, progressively writing 5+x, 9+x, 11+x, and 18+x to set up the proportion.

  2. 2:00 – 5:00 02:00-05:00

    The instructor writes the proportion setup '5+y : 9+y :: 11+y : 18+y' and the fraction form (5+y)/(9+y) = (11+y)/(18+y). Cross-multiplication is shown as '(5+y)(18+y) = (11+y)(9+y)', with the left side expanding to '90 + 18y' and then fully as '90 + 18y + 5y + y² = 99 + 99y + 11y + y²'. The equation simplifies to '90 + 23y = 99 + 20y', and the final value y=3 is calculated and boxed. A new problem with numbers 19, 37, 47, and 86 is briefly introduced before returning to the previous solution.

  3. 5:00 – 10:00 05:00-10:00

    The instructor demonstrates a shortcut trick using the numbers 5, 9, 11, and 18. The fractions 5/9 and 11/18 are set up for subtraction, with calculations '99 - 90 = 9' and '23 - 20 = 3' shown to find the difference. The result is used to form the proportion '9/3 = 3', marked with a checkmark. A general formula involving 'a, b, c, d' and the expression '|ad - bc| / (a + d)' is written down. A second worked example in red shows the question 'Que: What should be added to each of 19, 37, 47, 86 to make them in proportion?' with cross-multiplication '19 x 86 = 1634' and '37 x 47 = 1739', difference '1739 - 1634 = 105' underlined. The final frame switches to a new question: 'Que: What should be subtracted from each of a, b, c, d to make them in proportion?' with terms 'a-y, b-y, c-y, d-y' written.

  4. 10:00 – 13:47 10:00-13:47

    The video demonstrates the subtraction variant: 'What should be subtracted from each of 5, 9, 11, 18 to make them in proportion?' The instructor writes the expressions (5-y), (9-y), (11-y), and (18-y) and sets up the proportion equation '(5-y)/(9-y) = (11-y)/(18-y)'. Cross-multiplication gives '(18-y)(5-y) = (11-y)(9-y)', which is expanded and simplified to '-3y = 9', leading to the final answer y = -3. The instructor highlights the result and shows a verification step using ratios 2/6 and 8/15. The final slide displays the ratio equation in a red box with X marks, and a separate red circle contains the value '+3', followed by a 'THANKS FOR WATCHING' message.

The lecture teaches two methods for solving proportion problems of the form 'what number should be added or subtracted from four given numbers to make them proportional.' The first method is standard algebra: introduce a variable, set up the proportion by equating the ratio of the first two terms to the ratio of the last two, cross-multiply, expand, and solve for the variable. The second method is a shortcut formula involving |ad - bc| / (a + d) or related expressions, which allows faster computation. The instructor demonstrates both methods with the numbers 5, 9, 11, and 18 (yielding y = 3 for addition) and provides a second example with 19, 37, 47, and 86. The subtraction variant is also covered, showing that the same algebraic setup applies with (a-y) terms instead of (a+y) terms, yielding y = -3 (equivalently +3 in the final boxed answer). The key takeaway is that adding or subtracting the same number to all four terms preserves the structure of the proportion, and cross-multiplication is the primary tool for solving such equations.

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