An empty fuel tank of a car was filled with A type petrol. When the tank was half-empty, it was filled with B type petrol. Again when the tank was half-empty, it was filled with A type petrol. When the tank was half-empty again, it was filled with B type petrol. What is the percentage of A type petrol at present in the tank ?

Answer: Option B

Let the capacity of the tank be 100 litres.
Then,
Initially :
A type petrol = 100 litres
After first operation :
A type petrol = $$\left(\frac{100}{2}\right)$$ = 50 litres ;
B type petrol = 50 litres
After second operation :
A type petrol = $$\left(\frac{50}{2}+50\right)$$  = 75 litres ;
B type petrol = $$\left(\frac{50}{2}\right)$$ = 25 litres
After third operation :
A type petrol = $$\left(\frac{75}{2}\right)$$ = 37.5 litres
B type petrol = $$\left(\frac{25}{2}+50\right)$$  = 62.5 litres
∴ Required percentage = 37.5%