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Let the cost of a pen be Rs. X and the cost of a pencil box be Rs. Y .
Then, by given condition,
4x+4y=100=>x+y=25..........(i)
and. 3x=y+15
=> 3x-y=15............(ii)
On adding Eqs.(i)and (ii), we get
4x=40
=> x=10
By substituting x=10, in Eq (i) we get
y=25-10=15
Hence, the cost of a pen and a pencil box are Rs.10 and Rs.15, respectively.
⭐Mahir⭐
Solution :-
Let the cost of a pen be Rs 'x' and the cost of a pencil box be Rs. 'y'
As, the cost of 4 pens and 4 pencil boxes is Rs. 100
So, 4x + 4y = 100 .................(1)
And, three times the cost of a pen is Rs. 15 more than the cost of a pencil box.
So, 3x = y + 15
⇒ 3x - y = 15 ......................(2)
Now multiplying the equation (1) by 3 and equation (2) by 4, we get
⇒ (4x + 4y = 100)*(3) = 12x + 12y = 300 ................(3)
⇒ (3x - y = 15)*(4) = 12x - 4y = 60 ..........................(4)
Now, subtracting (4) from (3)
12x + 12y = 300
12x - 4y = 60
- + -
_________________
16y = 240
_________________
⇒ 16y = 240
⇒ y = 240/16
⇒ y = 15
Now, substituting the value of y = 15 in (2)
⇒ 3x - y = 15
⇒ 3x - 15 = 15
⇒ 3x = 15 + 15
⇒ 3x = 30
⇒ x = 30/3
⇒ x = 10
So, the cost of a pen is Rs. 10 and the cost of a pencil box is Rs. 15.