In what ratio should water be mixed with milk at Rs.29 per litre so as to get the average price of the mixture at Rs.25 per litre? (Assuming the cost price of water to be zero.)

Correct option is B

Given:

Rate of milk = Rs 29 per litre

Rate of mixture = Rs 25 per litre

Formula Used:

Cost of mixture =$$\frac{\text{Total Cost}} {\text{Total Quantity}}$$​​

Solution:

We know that price of water is zero

Cost of milk = Rs 29 per litre

Cost of milk = Rs 25 per litre

Let the ratio of milk and water be $$x:1$$​​

Then 

​$$(1 \times 0 ) + ( x \times 29 ) = (1+x) \times 25$$​​

​$$0 + 29x = 25 + 25x$$​​

​$$29x -25x = 25$$​​

​$$4x = 25$$​​

​$$x = \frac{25}{4}$$​​

The ratio of milk and water will be $$\frac{25}{4} :1 = 25 : 4$$

Alternate Method:

To find the ratio we use alligation rule:

Alligation rule:

Clearly the ratio of water to milk is 25: 4