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### Grade 8 - Mathematics2.12 Direct Proportion - I

 Two variables are in direct proportion to one another if they are always in the same ratio. If two variables x and y are vary in such a way that their ratio x/y is a constant(k). Then x and y are said to vary directly or in direct variation "k" is called the constant of variation. \ If variables x and y are directly then x/y = k or x=ky. This is denoted by x¥y. Example: y¥x, and constant of variation is 6. Find y, when x=3, find x, when y=24. Solution: Since y¥x, we have y = kx But k = 6 Þ y = 6x If x = 3 Þ y = 6*3 = 18 If y = 25 Þ 25 = 3x Þ x = 25/3. Example: If 4 bottles of milk cost \$2, how many bottles can you buy for \$8? Solution: You can write this in proportion as 4:2 :: x:8 or 4/2 = x/8 Cross multiplying we get 2x = 32 x = 16 Answer: 16 bottles Directions: Solve the following problems. Also write atleast 10 examples of your own.
 Q 1: y is directly proportional to x, and constant of variation is 15. Find x, when y = 30.264None of these Q 2: The toll on the Intercoastal Thruway is 8 cents for every 5 miles traveled. What is the toll for the trip of 115 miles on this road? (Hint: You are comparing cents with miles.)\$0.60\$2.33\$2\$1.80 Question 3: This question is available to subscribers only! Question 4: This question is available to subscribers only!