Question

The weight of 72 books of a certain type is 9 Kilograms. Then determine the weight of 40 such books?
72 books --- 9Kg.
40 books of the same type ---?

Answer 1

Hint – In order to determine the weight of 40 such books, we use the given data and calculate the weight that a single book of such type comprises. Using this relation we can find how much 40 books weigh.

Complete step-by-step answer:
Given Data,
72 books weight 9 kg.

Let us consider the weight of one book to be ‘x’ Kg.
The weight of one book is determined by dividing the total weight by the total number of books.
$
   \Rightarrow {\text{x = }}\dfrac{{9{\text{Kg}}}}{{72}} \\
   \Rightarrow {\text{x = }}\dfrac{1}{8}{\text{Kg}} \\
$

Now the weight of 40 such books would be nothing but the weight of one book multiplied by 40.
$
   \Rightarrow {\text{Weight of 40 books = }}\dfrac{1}{8} \times 40 \\
   \Rightarrow {\text{Weight of 40 books = 5Kg}} \\
$
Hence the weight of 40 such books is 5Kg.

Note – In order to solve this type of question the key is to know the concepts of ratio and proportion. By determining the weight of one book, we can calculate the weight of how many books by just multiplying them.
We did not convert the weight of one book into smaller units (grams) because keeping it in kilograms makes it easier for us to solve for any number of books.
It is important to always know the S.I units of quantities such as length, weight etc.