Class 8 Maths

# Linear Equations

## Exercise 2.2 Part 3

Question 12: Fifteen years from now Ravi’s age will be four times his present age. What is Ravi’s present age?

Solution: Let the present age of Ravi = m year
Age of Ravi after 15 years = m + 15 year

According to question, age of Ravi will be four times of his present age.
i.e. Age of Ravi after 15 year = 4 x present age of Ravi
⇒ m + 15 = 4 xx m
⇒ m + 15 = 4 m

After transposing m to RHS, we get
15 = 4m – 3
⇒ 15 = 3m

After dividing both sides by 3, we get

(15)/(3)=(3m)/(3)

Or, m=5

Thus, Ravi’s present age = 5 year Answer

Question 13: A rational number is such that when you multiply it by 5/2 and add 2/3 to the product, you get -(7)/(12). What is the number?

Solution: Let the rational number =a/b

As per question:

(text(Rational number)xx5/2)+2/3=-(7)/(12)

Or, (a/bxx5/2)+2/3=-(7)/(12)

By transposing 2/3 to RHS we get:

(a/bxx5/2) =-(7)/(12) -2/3

Or, a/bxx5/2=(-7-8)/(12)

Or, a/bxx5/2=-(15)/(12)

By dividing both sides by 5/2 we get:

a/bxx5/2÷5/2=-(15)/(12)÷5/2

Or, a/bxx5/2xx2/5=-(15)/(12)xx2/5

Or, a/b=-3/6=-1/2

Question 14: Lakshmi is a cashier in a bank. She has currency notes of denominations Rs 100, Rs 50 and Rs 10, respectively. The ratio of the number of these notes is 2:3:5. The total cash with Lakshmi is Rs 4,00,000. How many notes of each denomination does she have?

Solution: Let the number of Rs. 100 notes =2x

Number of Rs. 50 notes =3x

Number of Rs. 10 notes =5x

So, value of Rs. 100 notes =2x\xx100=200x

Value of Rs. 50 notes =3x\xx50=150x

Value of Rs. 10 notes =5x\xx10=50x

As per question, total cash =4,00,000=200x+150x+50x

Or, 400x=4,00,000

By dividing both sides by 400 we get:

(400x)/(400)=(4,00,000)/(400)

Or, x=1000

Substituting the value of x we can find the number of notes of different denominations as follows:

Number of Rs. 100 notes =2x=2xx1000=2000

Number of Rs. 50 notes =3x=3xx1000=3000

Number of Rs. 10 notes =5x=5xx1000=5000

Question 15: I have a total of Rs 300 in coins of denomination Re 1, Rs 2 and Rs 5. The number of Rs 2 coins is 3 times the number of Rs 5 coins. The total number of coins is 160. How many coins of each denomination are with me?

Solution: Given, total value of Rs = Rs 300
Total number of coins = 160
Coins of denomination = Re 1, Rs 2 and Rs 5
Number of Rs 2 coins = 3 x number Rs 5 coins

Let the number of coins of Rs 5 = m
Since, the number coins of Rs 2 is 3 times of the number of coins of Rs 5
Therefore, number of coins of Rs 2 =m\xx 3=3m
Now, Number of coins of Re 1 = Total number of coins – (Number of Rs 5 coins + Number of Rs 2 coins)

Therefore,
Number of coins of Re 1 = 160 – (m + 3m) = 160 – 4m
Total Rs = (Re 1 × Number of Re 1 coins) + (Rs 2 × Number of Rs 2 coins) + (Rs 5 × Number of Rs 5 coins)
⇒ 300 = [1 xx (160 – 4m)] + (2 xx 3m) + (5 xx m)
⇒ 300 = (160 – 4m) + 6m + 5m
⇒ 300 = 160 – 4m + 6m + 5m
⇒ 300 = 160 – 4m + 11m
⇒ 300 = 160 + 7m

After transposing 160 to LHS, we get
300 – 160 = 7m
⇒ 140 = 7 m

After dividing both sides by 7, we get

(140)/(7)=(7m)/(7)

Or, m=20

Thus, number of coins of Rs 5 = 20
Now, since, number of coins of Re 1 = 160 – 4m
Thus, by substituting the value of m, we get
Number of coins of Re 1 = 160 – (4 xx 20) = 160 – 80 = 80
Now, number coins of Rs 2 = 3m

Thus, by substituting the value of m, we get
Number of coins of Rs 2 = 3m = 3 xx 20 = 60
Therefore,
Number of coins of Re 1 = 80
Number of coins of Rs 2 = 60
Number of coins of Rs 5 = 20

Question 16: The organisers of an essay competition decide that a winner in the competition gets a prize of Rs 100 and a participant who does not win gets a prize of Rs 25. The total prize money distributed is Rs 3,000. Find the number of winners, if the total number of participants is 63.

Solution: Given, Total number participants = 63
Total prize money distributed = Rs 3000
Winner gets a prize of Rs 100
Loser gets a prize of Rs 25
Number of winners = ?

Let the number of winners = m
Since,
Number of winners + Number of losers = Total number of participants
Or, m + Number of losers = 63

By transposing ‘m’ to RHS, we get
Number of losers = 63 – m
Now, Total Prize money distributed to winners
= Number of winners X prize money distributed to each winner = m xx 100 = 100m
Total prize money distributed to losers
= Number of losers X prize money distributed to each loser
= (63 – m) xx 25 = (63 xx 25) – 25 m = 1575 – 25 m

Now, Total Prize money of winners + Total Prize money of losers = Total prize money
By substituting the total prize money distributed to winners and total prize money distributed to losers, we get
100 m + 1575 – 25 m = 3000
⇒ 100 m – 25 m + 1575 = 3000

By transposing 1575 to RHS, we get
100 m – 25 m = 3000 – 1575
⇒75 m = 1425

After dividing both sides by 75, we get

(75m)/(75)=(1425)/(75)

Or, m=19

Thus, number of winners = 19 Answer