Question

The cost of one pencil is Rs. 1.50, then the cost of 10 pencils is-
A. Rs 15
B. Rs. 150
C. Rs. 1.5
D. None of these

Answer 1

Hint: For this type of question unitary method is required and we must also know the multiplication involving decimal.Cost of one pencil is given, we have to find the cost of 10 pencils ,So multiply cost of one pencil with number of pencils to get the required answer.

“Complete step-by-step answer:”
Unitary method says that if the cost of x item is Rs. y then cost of 1 item is $\dfrac{y}{x}$ …(i) . Now if we wish to calculate cost of z items then it will be $z\times \dfrac{x}{y}$ …(ii)
Now we will use this principle in our question to calculate the cost of 10 pencils.
In the question we are already given the cost of 1 pencil so we don’t need to use equation (i) . Now we will use equation (ii) to calculate the cost of 10 pencils.
Therefore, our $\dfrac{x}{y}=Rs.\,1.50$ and z is equal to 10
Therefore, cost of 10 pencils is given by-
$\dfrac{x}{y}\times z=1.50\times 10$
Now, we need to multiply 1.50 by 10 which is done just like normal multiplication; only the decimal is put before the number of digits that were after the decimal in the numbers which are being multiplied.
Therefore, $1.50\times 10=15.0$
In this multiplication we can see that when we do $150\times 10$ , we get 1500 and then we put decimal before the last two digits because in 1.50 we have two digits after decimal.
Hence, the answer is Rs. 15.0 .
Note: We must know that if after decimal there is 0 it has no value, it means 15.0=15 and if 0 occurs at last in a number having decimal then again it has no value. Therefore, 1.50=1.5