Correct option is B
Given:
Ratio of alcohol to water = 4 : 7
Water is 9 litres more than alcohol
Solution:
Let the common multiplying factor be x.
Then, alcohol = 4x, water = 7x
Difference = 7x − 4x = 3x = 9
x = 3
Formula Used:
Water = 7x
x = 3
Water =
Thus, the correct option is (b) 21 litres
Alternate Method:

3 units = 9
1 unit = 3
hence water is 7 units then:
Water = 7 3
= 21 litres