Question : A 40-litre mixture of milk and water contains milk and water in the ratio of 7 : 3. How many litres of water should be added to the mixture so that the resulting mixture has 60% water in it?
Option 1: 30
Option 2: 35
Option 3: 20
Option 4: 25
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Correct Answer: 30
Solution :
Let the quantity of milk and water in the mixture be $7x$ and $3x$.
$7x+3x=40$
$⇒10x=40$
$⇒x=4$
Quantity of water in the mixture = $3x$ = 3 × 4 = 12 litres
Let $y$ litres of water is to be added.
According to the question,
$\frac{(12+y)}{(40+y)} \times 100 = 60$
$⇒\frac{(12+y)}{(40+y)} = \frac{3}{5}$
$⇒60+5y = 120+3y$
$⇒2y=60$
$\therefore y=30$ litres
Hence, the correct answer is 30.
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