In the verge of coronavirus pandemic, we are providing FREE access to our entire Online Curriculum to ensure Learning Doesn't STOP!

# Ex.8.3 Q7 Compairing Quantities - NCERT Maths Class 7

Go back to  'Ex.8.3'

## Question

(i) Chalk contains calcium, carbon and oxygen in the ratio $$10:3:12.$$ Find the percentage of carbon in chalk.

(ii) If in a stick of chalk, carbon is $$3\rm g,$$ what is the weight of the chalk stick?

Video Solution
Comparing Quantities
Ex 8.3 | Question 7

## Text Solution

What is Known?

(i) Ratio in which chalk contains calcium, carbon and oxygen $$(10:3:12).$$

(ii) Weight of carbon in stick of chalk $$(3\rm g).$$

What is Unknown?

(i) The percentage of carbon in chalk.

(ii) what is the weight of the chalk stick if it contains $$3\rm g$$ carbon.

Reasoning:

(i) To find the percentage of carbon, use the following formula

Percentage of carbon

\begin{align}=\frac{\text{Parts of carbon}}{\text{Sum of all parts}} \times 100\end{align}

(ii) the concept of percentage can be used find the total quantity.

Steps:

Parts of carbon $$= 3$$ and sum of all parts $$= 10 + 3 + 12 = 25$$

Percentage of carbon

\begin{align}&= \frac{{{\rm{Parts}}\;{\rm{of}}\;{\rm{carbon}}}}{{{\rm{Sum}}\;{\rm{of}}\;{\rm{all}}\;{\rm{parts}}}} \times {\rm{100}}\\&=\frac{{\rm{3}}}{{{\rm{25}}}} \times {\rm{100}}\\&= 12\% \end{align}

(ii) Carbon in the stick of chalk $$= 3\rm g,$$ and carbon percentage in chalk is $$12\%.$$

This means $$12\%$$ of carbon $$= 3\rm g$$

Let total quantity be $$Q$$

${\rm{I}}{\rm{.E}}{\rm{.}}\frac{{{\rm{12}}}}{{{\rm{100}}\;}} \times {\rm{Q}}\,{\rm{ = }}\,{\rm{3}}$${\rm{Total}}\,{\rm{Quantity}}\,{\rm{ = }}\,{\rm{3}} \times \frac{{{\rm{100}}}}{{{\rm{12}}}}\;\,{\rm{ = }}\,{\rm{25}}\,{\rm{g}}$

Learn from the best math teachers and top your exams

• Live one on one classroom and doubt clearing
• Practice worksheets in and after class for conceptual clarity
• Personalized curriculum to keep up with school