Explanation:

Correct Option - 4: 1:3

Given:

Price of the mixture per liter = Rs. 55
Price per liter of first type of milk = Rs. 70
Price per liter of second type of milk = Rs. 50
Calculation:
Let the quantity of first type of milk be x
Let the quantity of second type of milk be y
(70 x x) + (50 x y) = 55 x (x + y)
⇒70x + 50y= 55x + 55y
⇒ 15x = 5y
⇒x:y=1:3
:: The ratio of the two types of milk in this new mixture is x : y = 1:3.