The concentration of alcohol in Tequila is half of the concentration in Vodka. A cocktail consists of Tequila, Vodka and water only.
A: The amount of alcohol in a litre of cocktail, which consists of 50% Tequila and 30% Vodka
B: The amount of alcohol in a litre of cocktail, which consists of 30% Tequila and 50% Vodka

A: The amount of alcohol in a litre of cocktail, which consists of 50% Tequila and 30% Vodka

50% of 1L is 500ml, 30% of 1L is 300ml. 

Let us keep alcohol concentration in Vodka to be 2x%, tequila will be x%

So, $$\frac{x}{100}\left(500\right)+\frac{2x}{100}\left(300\right)=11x$$

B: The amount of alcohol in a litre of cocktail, which consists of 30% Tequila and 50% Vodka

50% of 1L is 500ml, 30% of 1L is 300ml. 

Let us keep alcohol concentration in Vodka to be 2x%, tequila will be x%

So, $$\frac{2x}{100}\left(500\right)+\frac{x}{100}\left(300\right)=13x$$