NCERT Book Class 8 Maths Chapter 11 Direct and Inverse Proportions

DIRECT AND INVERSE PROPORTIONS 129

CHAPTER
Direct and Inverse
Proportions
11
11.1 Introduction
Mohan prepares tea for himself and his sister. He uses 300 mL of
water, 2 spoons of sugar, 1 spoon of tea leaves and 50 mL of milk.
How much quantity of each item will he need, if he has to make tea
for five persons?
If two students take 20 minutes to arrange chairs for an assembly,
then how much time would five students take to do the same job?
We come across many such situations in our day-to-day life, where we
need to see variation in one quantity bringing in variation in the other
quantity.
For example:
(i) If the number of articles purchased increases, the total cost also increases.
(ii) More the money deposited in a bank, more is the interest earned.
(iii) As the speed of a vehicle increases, the time taken to cover the same distance
decreases.
(iv) For a given job, more the number of workers, less will be the time taken to complete
the work.
Observe that change in one quantity leads to change in the other quantity.
Write five more such situations where change in one quantity leads to change in
another quantity.
How do we find out the quantity of each item needed by Mohan? Or, the time five
students take to complete the job?
To answer such questions, we now study some concepts of variation.
11.2 Direct Proportion
If the cost of 1 kg of sugar is ` 36, then what would be the cost of 3 kg sugar? It is ` 108.