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Question

Cost of $$10$$ pens is thrice cost of two notebooks. Cost of one notebook and $$5$$ pens is $$60$$ rupees. 
What is the cost of $$7$$ pens ?  


A
30
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B
77
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C
63
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D
70
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Solution

The correct option is C $$63$$
Let cost of pen be $$Rs. x $$ 
and cost of notebook be $$Rs. y $$ 

Now, According to question, 
$$10x = 3 \times 2y $$ 

$$\Rightarrow 10x = 6y $$

$$\therefore y = \dfrac{10x}{6} $$    ...........(i) 

And, 
$$y + 5x = 60$$
$$\Rightarrow y = 60 - 5x $$  ...........(ii) 

From (i) and (ii) 
$$60 - 5x = \dfrac{10x}{6} $$ 

$$\Rightarrow\ 60 - 5x = \dfrac{5x}{3} $$ 

$$\Rightarrow\ 180 - 15x = 5x $$ 

$$\Rightarrow 180 = 20x $$ 

$$\therefore\ x = 9 $$ 

Cost of $$7$$ pen $$ = 7 \times 9 = 63 $$ 

Mathematics

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