7 litres mixture of milk and water contains 30% water. 3.5 litres of milk is added to this mixture. What is the percentage of water in the new mixture?

Options

A. 10
B. 15
C. 20
D. 25

Answer

To solve the problem step by step, we will follow these calculations:

### Step 1: Calculate the amount of water in the original mixture
The original mixture is 7 liters and contains 30% water.

\[ \text{Amount of water} = \text{Total mixture} \times \text{Percentage of water} = 7 \, \text{liters} \times 0.30 = 2.1 \, \text{liters} \]

### Step 2: Calculate the amount of milk in the original mixture
Since the mixture contains 30% water, it contains 70% milk.

\[ \text{Amount of milk} = \text{Total mixture} \times \text{Percentage of milk} = 7 \, \text{liters} \times 0.70 = 4.9 \, \text{liters} \]

### Step 3: Add the additional milk to the mixture
We are adding 3.5 liters of milk to the original mixture.

\[ \text{New amount of milk} = \text{Original milk} + \text{Added milk} = 4.9 \, \text{liters} + 3.5 \, \text{liters} = 8.4 \, \text{liters} \]

### Step 4: Calculate the total volume of the new mixture
The total volume of the new mixture is the sum of the original mixture and the added milk.

\[ \text{Total mixture} = \text{Original mixture} + \text{Added milk} = 7 \, \text{liters} + 3.5 \, \text{liters} = 10.5 \, \text{liters} \]

### Step 5: Calculate the percentage of water in the new mixture
The amount of water remains the same at 2.1 liters. Now we calculate the percentage of water in the new mixture.

\[ \text{Percentage of water} = \left( \frac{\text{Amount of water}}{\text{Total mixture}} \right) \times 100 = \left( \frac{2.1 \, \text{liters}}{10.5 \, \text{liters}} \right) \times 100 \]

Calculating this gives:

\[ \text{Percentage of water} = \left( \frac{2.1}{10.5} \right) \times 100 = 20\% \]

### Final Answer
The percentage of water in the new mixture is **20%**.

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