# Exercise 8.1 Comparing-Quantities -NCERT Solutions Class 7

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## Chapter 8 Ex.8.1 Question 1

Find the ratio of:

(a) $$\rm Rs\, 5$$ to $$50$$ paise

(b) $$15\,\rm kg$$ to $$210\,\rm g$$

(c) $$9 \,\rm m$$ to $$27 \,\rm cm$$

(d) $$30$$ days to $$36$$ hours

### Solution

What is Known?

Quantities in different units

What is Unknown?

Our task is to find the ratio of given quantities.

Reasoning:

These questions are based on the basic concept of comparing two quantities i.e. to find ratio. For finding ratio, first we should observe that both the quantities should be in same unit for e.g. we cannot compare metres with kilometres or grams with kilograms. So, we will have to convert both quantities into same units.

Steps:

(a) $$\rm{Rs} \, 5$$ to $$50$$ paise

Since, $$1 {\rm rupee} = 100 \,\rm paise$$

\begin{align}{\text{So, 5 rupees}}\,\,&= \frac{{{\rm{100}}}}{{\rm{1}}} \times {\rm{5}}\,\\&= \,{\rm{500}}\,{\rm{paisa}}\end{align}

Therefore the ratio

\begin{align}&=\frac{500}{50} \\ & =\frac{10}{1}\text{or }10\text{:}\,1 \end{align}

(b)$$15\,\rm kg$$ to $$210\,\rm g$$

Since$$, 1 \,\rm kg = 1000\, grams$$

\begin{align}{\text{So,}}\,\,15\,{\rm{kg}} &= \frac{{1000}}{1} \times 15\\ &= 15000\,{\rm{grams}}\end{align}

There for the ratio

\begin{align}&= \frac{{15000}}{{210}}\\ &= \frac{{500}}{7} {\rm{or}}\,\,500:7\end{align}

(c) $$9 \,\rm m$$ to $$27 \,\rm cm$$

$$1 \,\rm m = 100\, cm$$

\begin{align}{\rm{So,}}\,\,{\rm{9}}\,{\rm{m}} &= \frac{{100}}{1} \times 9\\ &= 900\,{\rm{m}}\end{align}

\begin{align}{\rm{Thus,}}\,{\rm{ratio}} &= \frac{{900}}{{27}}\\& = \frac{{100}}{3}{\rm{or}}\,\,100\,{\rm{:}}\,3\end{align}

(d) $$30$$ days to $$36$$ hours

Since, $$1$$ day = $$24$$ hrs

\begin{align}{\rm{So,}}\,{\rm{30}}\,{\rm{days}} = \frac{{24}}{1} \times 30\\{\rm{ = 720}}\,{\rm{hrs}}\end{align}

\begin{align}{\rm{So,}}\,{\rm{ratio}} &= \frac{{720}}{{36}}\\&= \frac{{20}}{1}{\rm{or}}\,\,20\,{\rm{:}}\,1\end{align}

## Chapter 8 Ex.8.1 Question 2

In a computer lab, there are $$3$$ computers for every $$6$$ students. How many computers will be needed for $$24$$ students?

### Solution

What is Known?

Number of computers for $$6$$ students

What is Unknown?

Number of computers for $$24$$ students

Reasoning:

This question can be solved using unitary method. Since number of computers required for six students are given, we can find how many computers are required for one student and from the answer, number of computers required for $$24$$ students can be calculated.

Steps:

For $$6$$ students, $$3$$ computers needed

So for one student computers needed

\begin{align}&=\frac{3}{6} \\ &=\frac{1}{2} \end{align}

And for $$24$$ students number of computers required

\begin{align}&=\,\text{24}\times\frac{\text{1}}{\text{2}}\, \\ &=\,\text{12}\,\text{computers} \end{align}

## Chapter 8 Ex.8.1 Question 3

Population of Rajasthan $$= 570$$ lakhs and population of $$\rm UP = 1660$$ lakhs. Area of Rajasthan $$= 3 \rm \,lakh\, km^2$$ and area of $${\rm UP} = 2\,\rm lakh\, km^2.$$

(i) How many people are there per  $$\rm km^2$$ in both these States?

(ii) Which state is less populated?

### Solution

What is Known?

Area and population of Rajasthan and UP.

What is Unknown?

Number of people $$\rm per\, km^2$$ and which state is less populated.

Reasoning:

To solve this question, we will divide total population with total area to know how many people are there $$\rm per\, km^2$$

Steps:

(i) Rajasthan. Number of people $$\rm per\, km^2$$

\begin{align} &= \frac {{570,00,000}} {{3,00,000}}\\ &= 190 \end{align}

It means $$190$$  people are there in $$\rm per\, km^2$$ in Rajasthan

UP. Number of people $$\rm per\, km^2$$

\begin{align} &= \frac {{1660,00,000}} {{2,00,000}}\\ &= 830 \end{align}

It means $$830$$ people are there in $$\rm per\, km^2$$ in UP

(ii) Which State is less populated?

Rajasthan is less populated because in Rajasthan less people live in $$\rm per\, km^2$$ as compared to UP.

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