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Let’s solve each question step-by-step.

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### Question 11
**Problem:** A mixture of 40 liters of milk and water contains 10% water. How much water should be added so that water may be 20% in the new mixture?

1. **Initial water amount**: 10% of 40 liters = 0.1 * 40 = 4 liters.
2. **Amount of milk**: 40 - 4 = 36 liters.
3. **Let \( x \) be the additional water to be added.** In the new mixture, total water will be \( 4 + x \), and total volume will be \( 40 + x \).
4. **Setting up the equation** for 20% water in the new mixture:
   \[
   \frac{4 + x}{40 + x} = 0.2
   \]
5. Solving for \( x \):
   \[
   4 + x = 0.2(40 + x)
   \]
   \[
   4 + x = 8 + 0.2x
   \]
   \[
   0.8x = 4
   \]
   \[
   x = 5
   \]

**Answer:** \( b. \, 5 \)

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### Question 12
**Problem:** One liter of water is evaporated from 6 liters of a solution containing 5% salt. The percentage of salt in the remaining solution is:

1. **Initial salt amount**: 5% of 6 liters = 0.05 * 6 = 0.3 liters.
2. **New volume after evaporation**: 6 - 1 = 5 liters.
3. **New salt percentage**:
   \[
   \frac{0.3}{5} \times 100 = 6\%
   \]

**Answer:** \( c. \, 6 \)

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### Question 13
**Problem:** To a sugar solution of 3 liters containing 40% sugar, one liter of water is added. The percentage of sugar in the solution is:

1. **Initial sugar amount**: 40% of 3 liters = 0.4 * 3 = 1.2 liters.
2. **New total volume**: 3 + 1 = 4 liters.
3. **New sugar percentage**:
   \[
   \frac{1.2}{4} \times 100 = 30\%
   \]

**Answer:** \( c. \, 30 \)

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### Question 14
**Problem:** The quantity of water (in ml) needed to reduce 9 ml shaving lotion containing 50% alcohol to a lotion containing 30% alcohol is:

1. **Initial alcohol amount**: 50% of 9 ml = 0.5 * 9 = 4.5 ml.
2. **Let \( x \) be the amount of water to be added.** New total volume will be \( 9 + x \).
3. **Setting up the equation** for 30% alcohol in the new mixture:
   \[
   \frac{4.5}{9 + x} = 0.3
   \]
4. Solving for \( x \):
   \[
   4.5 = 0.3(9 + x)
   \]
   \[
   4.5 = 2.7 + 0.3x
   \]
   \[
   1.8 = 0.3x
   \]
   \[
   x = 6
   \]

**Answer:** \( c. \, 6 \)

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Would you like further clarification on any question? Here are five additional related questions to practice:

1. If 50 liters of a solution has 30% acid, how much water should be added to reduce the concentration to 20%?
2. A 10-liter solution has 2 liters of salt. What percentage of the solution is salt?
3. How much pure alcohol should be added to 20 liters of a 40% alcohol solution to make it 50%?
4. If 5 liters of a 60% sugar solution is diluted by adding 2 liters of water, what is the new percentage of sugar?
5. A 15-liter solution has 10% alcohol. If 3 liters evaporate, what is the percentage of alcohol in the remaining solution?

**Tip:** To solve problems involving mixture percentages, carefully set up equations based on initial and final concentrations and solve for the unknown.

Answer the questions provided in the image on mixture and solution percentage problems.

Find detailed solutions to mixture and solution problems involving percentage concentration changes. Topics include calculating new concentrations after adding water or evaporating a part of the solution. Suitable for students in Grades 8-10.

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