A mixture of 30 litres contains milk and water in the ratio of 7 ∶ 3. How much water should be added to it so that the ratio of milk and water becomes 3 ∶ 7?

Step 1: Find milk and water in the original mixture

The ratio is 7 : 3, so total parts = 7 + 3 = 10 parts.

Milk = 7/10 × 30 = 21 litres

Water = 3/10 × 30 = 9 Litres
 So, initially:

Milk = 21 L
Water = 9 L

Step 2: Condition after adding water

When water is added, milk remains 21 L, but water increases.

Let the extra water added = x litres.

So, new water = 9+x

The required ratio is milk : water = 3 : 7.

That means, 

21/9+x=3/7

Step 3: Solve for x

21 × 7 = 3 × (9 + x)
147 = 27 + 3x
147 − 27 = 3x
120 = 3x
x = 40

We must add 40 litres of water.
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