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Question

A mixture contains alcohol and water in the ratio of $5 : 4$. If 9 litres of water is added to the mixture, the ratio of alcohol to water becomes $4 : 5$. Find the quantity of alcohol in the mixture.

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
20 litres

Problem Setup: Initial Mixture Ratio

Let the initial quantities of alcohol and water in the mixture be $5x$ litres and $4x$ litres, respectively, based on the given ratio of $5:4$.

Mixture Modification and New Ratio

9 litres of water is added to the mixture. The new quantities become:

  • Alcohol: $5x$ litres
  • Water: $(4x + 9)$ litres

The updated ratio of alcohol to water is $4:5$.

Calculating the Unknown Variable

We can set up an equation using the new ratio:

$ \frac{\text{Quantity of Alcohol}}{\text{Quantity of Water}} = \frac{4}{5} $

Substituting the quantities:

$ \frac{5x}{4x + 9} = \frac{4}{5} $

Cross-multiply to solve for $x$:

$ 5 \times (5x) = 4 \times (4x + 9) $

$ 25x = 16x + 36 $

Subtract $16x$ from both sides:

$ 25x - 16x = 36 $

$ 9x = 36 $

Divide by 9:

$ x = \frac{36}{9} = 4 $

Determining Alcohol Quantity

The initial quantity of alcohol in the mixture is $5x$ litres.

Substitute the value of $x=4$:

$ \text{Quantity of Alcohol} = 5 \times 4 = 20 \text{ litres} $

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Important Questions from Mixture and Alligation

  1. A bottle contains 20 litres of liquid A. 4 litres of liquid A is taken out of it d replaced by same quantity of liquid B. Again 4 litre of the mixture is taken out and replaced by same quantity of liquid B. What is the ratio of quantity of liquid A to that of liquid B in the final mixture?

  2. The average score of a batsman after his 50th innings was 46.4. After 60th innings, his average Score increases by 2.6. What was his average score in the last ten innings?

  3. If 1 litre of water weighs 1 kg, then how many cubic millimeters of water will weigh 0.1 gm?

  4. A vessel full of water weighs 4o kg. If it is one-third filled, its weight becomes 20 kg. What is the weight of the empty vessel?

  5. Two equal glasses of same type are respectively 1/3 and 1/4 full of milk. They are then filled up with water and the contents are mixed in a pot. What is the ratio of milk and water in the pot?

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