DRDO Multi tasking staff Model paper 1

Tips & Tricks
Short trick:
Alcohol ∶ (Cold drink + Water) = 1 ∶ 3
∴ Amount of Alcohol = 1/4 × 28 = 7 L
Detailed solution:
We have a 20L mixture of alcohol and water.
⇒ A + W = 20 ---- (1)
Let the cold drink be denoted by C
According to question, A : (W + C) = 1 : 3
⇒ A/ (W + 8) = 1/3
⇒ 3A = W + 8
⇒ W = 3A – 8 ----(2)
From (1) and (2)
⇒ 4A = 28
⇒ A = 7
Hence 7L of alcohol is present in the mixture.