In a mixture of 45 litres the ratio of milk to water is 4:1. Additional 21 litres of water is added to the mixture. Find the ratio of milk to water in the resulting mixture. Choose the correct option
A) 2 / 1 B) 4 / 1 C) 6 / 5 D) 3 / 4 E) 3 / 2

Rationale: "given that milk / water = 4 x / x and 4 x + x = 45 - - > x = 9 . thus milk = 4 x = 36 liters and water = x = 9 liters . new ratio = 36 / ( 9 + 21 ) = 36 / 30 = 6 / 5 . answer : c ."
Answer: C