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  1. Chapter 3 Class 10 Pair of Linear Equations in Two Variables
  2. Serial order wise
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Ex 3.7, 4 (Introduction) The students of a class are made to stand in rows. If 3 students are extra in a row, there would be 1 row less. If 3 students are less in a row, there would be 2 rows more. Find the number of students in the class. Introduction Let 2 Students stand in 1 row and there are 10 rows in total Total the number of Students = Number of Students in 1 row × Number of rows = 2 × 10 = 20 Students We will use the formula Total number of Students = Number of Students in 1 row × Number of rows Ex 3.7, 4 The students of a class are made to stand in rows. If 3 students are extra in a row, there would be 1 row less. If 3 students are less in a row, there would be 2 rows more. Find the number of students in the class. Let number of students in 1 row = x and number of rows = y Then, Total number of Students = Number of Students in 1 row × Number of rows Total number of Students = xy If 3 Students are extra in a row, there would be 1 row less. Hence, If 3 Students are less in a row, there would be 2 row less. Hence, Number of students in 1 row = x + 3 Number of rows = y − 1 Now, Total Number of Students = Number of Students in 1 row × Number of rows = (x + 3) (y − 1) Putting number of Students = xy xy = (x + 3) (y − 1) (x + 3) (y − 1) = xy x (y − 1) + 3 (y − 1) = xy xy − x + 3y − 3 = xy − xy − x + 3y − 3 = 0 x − 3y + 3 = 0 Number of Students in 1 row = x − 3 Number of rows = y + 2 Now, Total Number of Students = Number of Students in 1 row × Number of rows = (x − 3) (y + 2) Putting number of Students = xy xy = (x − 3) (y + 2) (x − 3) (y + 2) = xy x (y + 2) − 3 (y + 2) = xy xy + 2x − 3y − 6 = xy 2x − 3y − 6 = xy − xy 2x − 3y − 6 = 0 Hence, equations are x − 3y + 3 = 0 …(2) 2x − 3y − 6 = 0 …(3) From equation (2) x − 3y + 3 = 0 x = 3y − 3 Putting x = 3y − 3 in equation (3) 2x − 3y − 6 = 0 2 (3y − 3) − 3y − 6 = 0 6y − 6 − 3y − 6 = 0 3y − 12 = 0 3y = 12 y = 12/3 y = 4 Putting y = 4 in equation (2) x − 3y + 3 = 0 x − (3 × 4) + 3 = 0 x − 12 + 3 = 0 x − 9 = 0 x = 9 Therefore, Number of Students in 1 row = x = 9 Number of rows = y = 4 Total Number of Students = Number of Students in 1 row × Number of rows = 9 × 4 = 36

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    A man travel 370km partly bytrain and partly by car.if the cover 250km by train and test by car it take him 4hr but if he travel 130kmby train and restby car it take 18mins longer find the speed of car and that of train

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