Direct and Inverse Proportions
CBSE Class 8 - Mathematics
Q1: If x and y are directly proportional and when x = 13, y = 39, which of the following is not a possible pair of corresponding values of x and y ?
(a) 1 and 3
(b) 17 and 51
(c) 30 and 10
(d) 6 and 18
Answer: (c) 30 and 10
For direct variation,
x1 | = | x2 |
y1 | y2 |
Thus x/y = 13/39 = 1/3. Options (a), (b) and (d) have the same ratio while option (c) does not.
Q2: Which of the following is in direct proportion?
(a) One side of a cuboid and its volume.
(b) Speed of a vehicle and the distance travelled in a fixed time interval.
(c) Change in weight and height among individuals.
(d) Number of pipes to fill a tank and the time required to fill the same tank.
Answer: (b) Speed of a vehicle and the distance travelled in a fixed time interval.
∵ Speed / distance = fixed ratio.
Q3: Both u and v vary directly with each other. When u is 10, v is 15, which of the following is not a possible pair of corresponding values of u and v?
(a) 2 and 3
(b) 8 and 12
(c) 15 and 20
(d) 25 and 37.5
Answer: (c) 15 and 20
u/v = 2/3
Option (C) does not maintain the same ratio.